They are a kind of tiny white worm, I think planaria. They can be introduced by plants or other fish, I think. As recommended, do a pwc--wipe down the tank walls first with a clean sponge so you can vac them, too. It may take a couple of times to get rid of them, but they don't really cause any harm and the fish eat them. I had them in my fry tank at one point as I was trying different foods when they outgrew the fry powder, and there was an excess of food after two days!! All gone, no recurrence.