If the snail consumes that stuff, I doubt that’s Dinos.
Dinos can be toxic.
3 weeks, very normal uglies stuff, not much their, makes more sense that’s it’s Diatoms this early.
brush it off, suck it out, you need to be the cleaner until your bacteria populations are sufficient to eradicate that stuff.
keep every parameter in the “zone” with as little change as possible. The less change in those, the faster your creatures develop and with increased diversity.
Always keep your nutrients available or stuff starves and develops very slowly, and because of that, pest type algaes like diatoms, Dino’s and Cyano can take over.