Bristle worms, they are cleaners/eat fish poo, left-over foods, etc. as long as you dont have small fish they are OK.
if a small fish trys to eat them they can lodge in their throat or stomach or intestines (due to the bristle type hairs they have), and starve your fish and kill em'.
if you wat to get rid of some of them, they make traps, or you can buy a 6-line wrasse and he will eat em'.
in almost every case, if you see one, there are 50 you dont see!!!