This sent me down a rabbit hole of trying a lot more gibberish/unconventional prompts on Midjourney. There’s something fascinating about seeing such a complex response that’s clearly an unintended side-effect of the training process, even though the results have a pretty conventional “Midjourney look”.
Putting in your username is also an amusing exercise. This is what I get for babelspace The ‘space’ influence is pretty obvious, I think it’s associating babel > babble and babies, which I assume is where the color scheme and youthfulness comes from.
If anyone without a Midjourney subscription wants to see what their username looks like, I would be happy to run it for you.

That's neat!

To me the common element in your picture seems to be some sort of orange teardrop shape. In some cases it’s a balloon, in others it’s a mushroom.

Decided I would try this with Stable Diffusion - Emaonly 1.5

::: spoiler results TUPTLE CRAPLIU -

Tiupli - bullshit

CHRMLLNIA - :::

So yeah, I don't think SD has anything of this sort. The results are similar to if I type in complete bullshit: OPSKAEOPSAEKSAELMAESOJSAEKLHWQEWQECOIUWEOIWQODWDJHSDMWOIEUQWRRNFJKBEIURFYHQOEIEJFOLEIFDJALSKDJPAWOJODIJDIWHFN

::: spoiler results :::

Also typed in my username, as some commented in the thread. I got a lot of pictures of singers who looked suspiciously like Abel Tesfaye. My other username, which obviously has "deer" on it:

::: spoiler results :::

My old discord username, which has nothing to do with buildings whatsoever:

::: spoiler results :::

So... I don't know that much about it, but as I understand it, this isn't really a "hidden" language in the sense of something deliberate, but more the outcome from stimulating the language-understanding network in a way it's not been trained on. If it gets a string of tokens that's just some nonsense, then it'll tend to light up its output nodes in some semi-random pattern that's nonetheless slightly consistent depending on what the input is. Then, that somewhat-random pattern of nodes will go into the art-generating network, and stimulate some random set of nodes related to what art it wants to produce, without adding up to anything coherent. And yes, the results (if e.g. you feed it just a string of gibberish from a nonsense word generator) are fairly wonderful:

I typed in more of the weird text from the logos with the following results:

Prompt:

FROTIE FIOLE --q 2

Output:

Prompt:

NOTE MAMESE --q 2

Output:

Conclusion:

While these are all beautiful images, they have nothing to do with either the original fitness prompt or the logos. Though the first prompt seems to consistently mean "frog", and the second one "a girl playing music".

For both of these prompts, I received a warning from Midjourney:

Action needed to continue

Sorry! Our AI moderators feel your prompt might be against our community standards.

If you think this is a mistake, please press the "Appeal" button below and we will send it to a more sophisticated AI to double-check the result. /imagine NOTE MAMESE --q 2

And I had to appeal. I won't do any more of these experiments because I don't want them to ban me :D