That sounds like a specie of Eunice worm. I had a pretty big one that I kept in my supmp for a couple years. It was black with tentacles on it's face / mouth area that were reddish / orange in color. It had a huge burrow that it made in the sand, it was pretty cool.
The only thing I know it was eating was calerpa, I would see it drag pieces back to it's hole. I had coral frags down there with it and didn't have them get eaten so I assumed it was not a coral eater. It looked pretty scary though, so I wouldn't touch it
It disappeared one day, don't know if it died or if it went through a pump or what - neat but I wouldn't want it in my display tank. I have read stories where species of these are coral predictors.
Joel