I doubt Germans or Danes would get the hint that "midnight" is a stand in for adults only content. Mitternacht is more for ghost stories.
For English speakers and mostly in the UK, you might have used blue as a code word. Such as in Blue Movies.
For the germanic languages you might use a direct approach, if you want to spell out in your account name, what you sell. But I guess that this will have zero influence either way. People will not search your content by your user name. If they specifically search for it, they might use
https://itch.io/game-assets/tag-adult/tag-ai-generated
Ther are only 100, and 5 of them are yours.
But look at your anayltics, maybe you find inspiration there, how you can get found more easily by people looking for your content.