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That is indeed a bristle worm. Most likely hitchiked into the tank from some of your live rock. They live inside the rock or in the sand bed. Quite beneficial to the tank. Sometimes its the little stuff that we find in our tanks that excite us more than the big stuff.
My lfs recommends removing the larger ones and leaving the smaller ones because they eat detritis. I look once in awhile after dark and take out any large ones I see with a pair of hemostats. They don't hurt anything, but you might want to do some population control.
Bristle worms are harmless. I have heard people blast them like their a plague but my experience and that of anyone else I have spoken to is that they pose no threat to you tank. I have several in my 30g reef that have grown to over 10 inches over the 5 years it has been set up and they do nothing but clean the crap out of the sand. I would leave him in their personally, IMHO anyways