yup they are bristle worms
but u may want to know that even though they are good for your tank thats only in smally amounts. so if u start to count more than 20 u better start getting rid of them. when they run out of food they will sometimes eat corals and they will also kill sick inverts like clams or other sesile creatures.
they are easy to get rid of... eather a stor bought trap or some fish and inverts eat them; six-line wrasse, arrow crabs, coral banded shrimp, and some sargant major damsels.