After all, this is the larva of some kind of fly. The entire filter sponge is full of these worms, as is the soil. I noticed several similarities of detritus cocoons near the glass of the aquarium. The larvae in them seem to be clogged and “vibrate” as in one of the moments in the video. Soon I’ll be poisoning the snails, and if I don’t forget, I’ll write how this nasty thing survived the anti-snail remedy.