Are you asking a question? Remember that questions on this site need to be specific, not open ended discussion prompts. I searched your whole post and I don't see a single question mark.

@YowaneHaku If that was the case then OP could make a quick edit to their question and it would still not be a good fit for this site given our no questions about existing works of fiction policy and our one question per post policy. We also require that questions be sufficiently specific to not have many valid answers. If we're charitable and allow for posts not asking a single question on a question and answer site, this is still far more of a request for brainstorming and idea generation than this site permits.

@sphennings There is an anatomically correct King Kong as well as many other mythical creatures. This does not at all break any third party world rules, because the world these creatures occupy is Hell, of my own design. References to Lovecraft’s Mi-Go are for descriptors only. They are not, in fact, the god-like race of the Mi-Go. As Yowane Haku stated, the physical traits required of the creatures are well described. The “question mark” is: “How could an anatomically correct creature exist that fits the description of a Mi-Go?”

@JBH I am not in any way building on any existing world. Lovecraft’s worlds are filled with gods. He directly defies science in his horror.

@sphennings The problem statement is in the first paragraph, second sentence: “ I now need to try to come up with an anatomically correct alien species that fits the description, in more modern terms.” You can quickly re-arrange this into a question by changing “I need to...” into “How can I …”

@VogonPoet The King Kong Q probably asked before the 3rd party rule was formalized under current rules that Q would not be permitted as written. If you want to ask about your Mi-Go knockoffs, that would be permitted but you are required to "file off all the labels" and describe the creatures in the body of the question, without relying on 3rd party canon. Then you need to ask a single specific question, one that isn't brainstorming or fishing for ideas.

Going from saying "I need to try to come up with an anatomically correct alien species that fits the description, in more modern terms." to asking "How can I try to come up with an anatomically correct alien species that fits the description, in more modern terms." will still leave you with a question that isn't a good fit. There are many different methods of coming up with aliens each are equally valid. If you asked how to make an "anatomically correct alien" you'd then need to explain what you mean by AC in the context of a Mi-Go. Otherwise you're just fishing for ideas, which isn't ok.

If you want to salvage the Q. Read up on the current meta discussions about 3rd party worlds.

Within that you can get a good idea of the standards of review for any question, and why we long ago decided to prohibit Qs about 3rd party worlds.

As it's currently written your Q has a few (potentially salvageable) issues to it.

1. Don't mimic the anatomically correct format if you're not going to follow it's rules. This is especially true now since we've has a slew of very low quality ACS questions and are considering preventing any future asks in the format.

2. Make your question clearly about your world. It's generally a good idea to file off the labels as much as possible. It's always been OK to ask about your laser sword, but not as much to ask about the lightsabers of Star Wars.

I am fairly certain that mentioning third party works for the sake of clarity and brevity is perfectly OK per the lengthy discussions over TPW. In fact, my Hell world uses this same language. But just the same, I did clarify what was meant by “can not be photographed,” how they may travel, & how they may find energy. Also, this creature really can’t be divided into separate questions because the method that grants them space mobility, levitation, & obscurity are very likely one and the same.

I did look over the “anatomically correct” series description closely before posting, and I feel I stayed within the guidelines.

3. Follow other site policy about asking questions. Make your question specific. Provide all necessary information. Don't ask an open ended question. Don't ask questions with many valid answers. Don't ask us to establish facts about your world for you. Don't ask questions that rely on opinions to be answerable.

You mention 3 major characteristics. Try asking about one of them at a time.

The only thing I know of that allows for all 3 is magic, or something vast and unknowable beyond the stars.

Even if you can convince us that it is actually one ask, that has nothing to do with their anatomy per-se, you'd be asking us to come up with some convincing bio-babble to explain their magic powers.

Call it a vogon's organ, say that it makes them levitate, and be unphotographable, by absorbing the poetry of the angles at the bottom of the Mandlebrot set.

Or wiggle your fingers and say "magic", "super advanced alien technology", "super advanced alien biology", or "The horror, the horror, even the opium is no longer to keep the horror of knowing at bay"

But to have a Q with many valid answers, is to have something that is not suitable for this site.

I don’t think I did any of those things, which is why it is words the way it is. “Specific” = fungal creature existing on ice planet derives energy from radiation, travels through space by scientifically feasible means if possible. Only a very small subset of answers can solve this problem. Squid jets? Nope - in a vacuum. Propellers? Nope. Jet engines? Nope. There i are only a few valid answers. And magic is properly scoped: “Only what is impossible with known physics is of supernatural origin.”

As for how to avoid filing off the labels, you need to be very clear that you're asking about your Mi-Go knockoff not the works of Lovecraft. Best to do more than just tell us this, edit your question to show this fact to us as well. Be clear how your creature differs, what you've established about your version already.

Perhaps in your world there's a luminiferous ether that they can swim through.

There are many ways to attempt to explain any behavior by science.

Your question is underspecified and underconstrained.

If you know of some valid answers why are you asking us?

That makes it seem like you're wanting to prompt discussion, rather than solve some problem you're having with your world.

Every problem-question has a plurality of answers. It is the job of the forum to score the best fit, not to decide the “singular possible solution.” Even strict coding stack exchanges have thousands of possible solutions to a programming problem. Clearly, the answer to this problem involves either NO MAGIC AT ALL or the absolute minimum violation of physical laws. So the “luminiferous ether” and other such nonsense answers are automatically rejected.

What particular issues are you having with your critter. Ask us for help with that, rather than asking us to build your critter for you.

Technically it's one violation of physical laws to say "In my world X critter just exists"

Currently you're asking us to build your critter for you.

Instead try to build your critter yourself. If you encounter a problem, ask us to help you solve that far more specific ask than your current "Build my critter for me".

This sound more like you simply dislike the anatomically correct series as a whole. I don’t agree with any of the purported policy violations here. Yes, please explain the anatomy of a critter that fits the description of a Live raft ian Mi-Go. What is the skin made of to cause light aberrations? What is the body mass that causes it’s almost-levitation? What causes propulsion in a vacuum? As separate questions the mongrel that comes out will be completely unusable!

You can say "But my Q is OK" as many times as you want but, 5 people including a mod voted to close it, and it has been rejected from the VTRO queue.

Try a little experiment, try to build your critter yourself. If you encounter a problem, ask us to help. See if that question stays open.

Then you're asking us to build your world for you, which we do not do on this site.

We're here to solve specific problems, not brainstorm or generate ideas. It seems like you're wanting us to brainstorm and generate ideas for you.

I did try. I genuinely don’t know how to make something that obscures photographic film. Prisms? Coherent refraction? Subdermal Holographic 3D etchings? I put the limits of my knowledge in the question: Ion drives, radioactivity, etc.

That's a more specific ask than "What would make something unphotographable?" since only answers that work with your particular choices for propulsion, and absorption would be valid.

Keep in mind that you now have an answer to your Q so you're restricted in how you can edit due to our policy of not invalidating existing answers.

I should also point out how by saying your knockoff uses Ion Propulsion or whatever, you're showing us (as opposed to just telling us) how your critter diverges from canon. For instance I say that "my lightsaber works by containing plasma" then ask about the energy requirements to be able to generate a blade and slice through a door, I'm showing you how my lightsaber works meaning that answers don't need to rely on canon, or make up their own explanation for how it could work, before answering.

As opposed to just asking "What are the energy requirements for a lightsaber to be able to generate a blade and slice through a door?" without providing any additional context.