Part of your problem is that half the folks posting to those subs have no idea what “moe” means either. Folks, it’s not supposed to be a catch-all term for sexy anime women!

Everyone has their own definition of moe, and I’m far from an expert because it’s not my thing, but my understanding (based on how the word was used in the US and in Japan during the 2010s anyway) is this: It’s kinda like the protecc meme… Something that is moe is usually cute, small, maybe a bit defenseless or derpy. It’s something that causes a feeling of “I love and want to protect this cute little thing”… Like how you might feel about a puppy, especially if the puppy is doing something cute and derpy like a heckin’ protecc (in normal people speak: something that’s adorable because it’s not especially exceptional but it feels exceptional when the cute little thing does it, like a puppy that thinks he’s protecting his mama when he barks at a butterfly and then looks pleased with himself when it flutters away, which causes you the viewer to say “d’aw aren’t you just the best little guard dog, yes you are!”). Except puppies are not usually considered moe as moe has a sexual connotation because it’s an otaku thing so of course it does. In other words it’s a term with a pretty strong loli vibe (up until very recently I had never seen it used in reference to anything but girls or maybe women with girlish traits, or girl-like creatures, like monster-musume). Moe got especially big around the time K-On came out, and for a while that series was heralded as the ultimate moe anime: cute girls doing cute things. It’s kinda like the daughter version of “waifu.” And some people do use the term in a more innocent, non-sexual way, but then there’s the hentai doujinshi and body pillow crowd that taint the entire concept (as with all things anime).

Therefore it’s really fucking confusing to see these “moe” communities filled with adult-presenting anime characters sporting G-cups and “fuck me” eyes. Maybe some moe communities use the term correctly, again it’s not my thing so I don’t go searching for it, but the ones I’ve stumbled across in the deep recesses of all sure don’t seem to fit the bill. Am I just old and out of touch with how the kids use their Japanese-derived slang nowadays? Did the normies start altering the use of otaku terms after anime became mainstream? Eh, maybe. But for at least a solid decade moe had a specific, albeit poorly defined, meaning.

Edit: I generally agree with loppy’s comment, in that what they describe is a technically more correct and authentic way to use the term “moe.” However it’s a situation akin to how most people use “decimate” to mean “obliterate,” even though it technically means “to reduce by 1/10th.” IMO what I describe is how most/nearly all (Western) otaku and weebs would describe/use moe, while loppy’s over in the corner going “actually…” In other words, what I’ve described is the “normie otaku” definition, and loppy’s is the “otaku of otaku” definition. So I guess that means that I’d still consider my answer “correct” because it’s the more common definition, even though it’s technically wrong.

It’s hard to describe.

A common misconception is that moe (pronounced not like the name “Moe” but more like “moh eh”) is a description of something, like how the word “cute” describes a certain way that something looks. Instead, moe is a feeling you experience, and you say that something “is moe” if looking at it makes you feel the “moe” feeling. You also say that someone “has a moe for” something if it’s something that often/usually makes that person feel moe. I don’t think moe is a “strange” feeling, I think it’s something most people experience but they just don’t describe it in this way.

Another common misconception is that moe is sexual in any way, but it emphatically is not. That doesn’t mean moe can’t coincide with sexual feelings, but more often than not it doesn’t. Actually, I might partly describe “having a moe for something” as a nonsexual fetish.

A commonly cited etymology is the Japanese word “moeru”, meaning to burn up (though I don’t know how accurate that is). Moe is an intense feeling of passion for something, often said to involve a desire to “protect”. This doesn’t necessarily mean “protect from harm”; often it’s much more like “I don’t want this thing to ever disappear”.

Another misconception is that “thinking something is moe” means “it’s cute”. Moe doesn’t necessarily have anything to do with cuteness. As an example, someone could have an intense moe for glasses characters, meaning essentially they really love characters with glasses regardless of anything else like how cute they are. I think this cuteness misconception comes from the proliferation of a certain type of anime around the late 2000’s/early 2010’s, focusing mostly cute girls not doing much else than being cute. These shows were certainly intended to make their audience feel moe, and the word was applied a lot to describe them, and so people unfamiliar with the word naturally though “moe” described these shows specifically.

Moe is often associated with anime/manga/etc. and you could reasonably restrict it to apply only to this sphere by definition, but I don’t think this is necessary. But because of that it usually designates a space as anime-related.

So these “X-moe” communities are communities for sharing images that someone who has a moe for X would enjoy, and also implicitly these are “anime-esque” images because “moe” as a word sort of codes for those sorts of communities.