Hair Like Free Swimming Worms. SOLVED.

They are harmless. Most people call them planaria, though Richard W said a while ago now that they aren't, they're actually detritus worms.

They usually live in the substrate but move into the water if there is too much food causing a population explosion. Curing that usually cures the worm over-population. They aren't much of a problem in fish tanks as fish will eat them, but I don't think shrimps will.