Could be a Bobbit Worm. ( Eunice Aphroditios ) These worms can be pretty mean and stay rather slim but grow quite long and come in off of live rock. They have tentacle like mouths. I had a small one years ago and I put him in his own tank. Very interesting to watch.
You Tube has a video of someone feeding one. They are kind of like the mantis only in worm form.
hth
http://www.youtube.com/watch?v=tkUSx...eature=related