Worm Shaped Critter - What is it?

[RYcube2951]

Found these worm type critters in my substrate during water change. The body is long and slender with a dark colored stripe on it’s back. Has anyone had these and can identify what they are and if harmful to corals, fish or inverts? Thanks.

 

[shakacuz]

bristle worm. good clean up crew but ugly. do not touch with bare hands

 

[ying yang]

Looks like baby bristle worms and most class them as beneficial cuc members,I've found a few small ones like yours in my tank .
Aslong as they stay small I don't mind them,but if they get big beasts like 6 inch plus then I don't like them and don't want in my tank ha ha