I had one come on live rock too. UGLY critter. do a search on Bristle worm and see if that's what you have. Benefical.....I guess. to me...UGLY. I flushed it!!!
They are ugly. The guys at my LFS were cleaning/rearanging one of their oldest tanks the other day, and one of the guys found one about 4 feet long! I thought it was f-ing nasty.