It's an amphinomid and most likely Eurythoe complanata. An up-close view of the head is needed to be sure it's not the coral-eating species Hermodice. Eurythoe is a scavenger & detritus eater.
Thanks for the new pics. It's Eurythoe and not a problem unless you just don't like the way they look. As many will tell you they are beneficial scavengers & detritivores and help keep tanks clean. They won't disturb healthy clams but if the clam is dying the worms will be attracted to the dead tissue.
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