My vote is a detritus worm. If you have too many than reduce feeding or clean your substrate more often if possible.
If your in the hobby long enough you'll always end up with some sort of detritus worms. Mine are much shorter but if I overfeed they multiply like mad. Killing them typically requires harsher meds that really isnt a good idea with planted tanks or inverts. Afterall they just eat what your fish leave behind. Now if they had a diamond shaped head and appeared to be flat and stuck to the glass, then it would be Planeria, which are definitely bad to shrimp if you have any. But that's not what you have so yeah, no panic.