Worms? Leeches?

From zooming into the photo, those are planarian (basically little worms that are harmless but have the appearance of a tiny white leech). They are not snails who left their shells (no freshwater snail does this), and they are not harmful.

You have them naturally as they are attracted to excess waste/food being in the tank. I’d recommend feeding about half of what you currently do and they will reduce/disappear (from above the substrate level) once that is done.

Oh yeah, I used to get those when I started a lot because I was letting waste build up too much.