If it has a burrow it's a scavenger and nothing to be concerned about. That it lives in a rock and grabs dead shrimp bits suggests that it is such. Ours is about 1/4 inch in diameter and happens to be my girlfriends favorite tank resident. We have seen a few juveniles poke their heads out so hopefully I will have some for sale eventually. My species is an attractive green and definitely a Eunicid with the 5 anterior tentacles and the really neat eversible pharynx.
Here is the article:
http://www.reefkeeping.com/issues/2003-04/rs/index.php