Bristle worm. It's harmless. If you see an abundant amount, it may aggrevate your anemones (if you have any). They have fish that will eat them. Other than that, you are fine with them.
i second it.. bristle worm.. if you have one your probably have lots more too
grab a flashlight and come in around 2 am after lights have been out a while and I bet you'll see the entire community!