Well, it definately sounds like a bristle worm. On reef central there was a guy who had a 500 gallon tank who had to take his whole tank apart because he had a 6 foot bristle worm in it! There are pics too. I will see if I can find a link. Once they get that large (12"+), you really should remove them IMO.
EDIT:
Found them!
http://reefcentral.com/forums/attach...postid=1032288
http://reefcentral.com/forums/attach...postid=1033108
Yea... pretty nasty!