Brisleworms eat detritus. Likely entered your tank in your rock. They are only an issue if you overfeed your tank, that will increase the population by a bunch. They won't harm your fish or corals.
..just don't TOUCH them. Those bristles hurt when they get stuck in your finger.
They can be pink or pink/gray. Nothing you need to be overly concerned about.
You can dump him in your sump and he can eat food that manages to get through to there.
That made me almost throw up. I HATE bristle worms but I leave them be cause I've never had a serious issue with them. That was a disturbing video. The worst is NO GLOVES!