Those are little planaria worms.. pretty harmless, but annoying.
Having an outbreak like that suggests that there's likely too much waste food falling to the gravel, and gunk and poo in it as well. So cup your fishy, get a gravel vac, do a big water change and swish your gravel all about, suck all the filth out, rinse repeat if necessary, and that'll solve it, more than likely. Clean your gravel with a siphon regularly. And don't overfeed.
Flake food is -great- at creating a nice gunky home for these guys. Pellets, not so much.
Love your fishy btw, so adorable! The planaria won't hurt it, they drop off after a while.