looks like a simple bristle worm to me, my tank is riddled with them and I have all types of corals except leathers and some more rare types. I think, and im not expert by any means, your fine. They are mostly scavengers that clean up debris and excess food at night. If you have liverock from another tank you most likely already have them and dont know it.
Some people HATE bristle worms and get them out as much as they can, I on the other hand havent ever had a problem. Im sure someone with more experiance here will chime in. Also a good thing to live by "When in doubt, throw it out." Can't go wrong there.
