With AI upscaling it fills it in based on the training from other images/videos. So it probably won’t be an alien, but small details common in other videos that looked similar will also show up in the upscaled videos. If an extra flower shows up in a field of grass it’s usually not a big deal, but for some things like faces or symbols, small details can really change the way people interpret it.
The creator of pixelfed is working on a tiktok alternative loops, although for now it’s in private beta.
For a starting point that is available now, you could look at Pixeldroid, an open source pixelfed app.