I’d guess a Syllid worm (taxonomic family Syllidae) of some kind. If it is a Syllid worm, then there’s a good chance it’s safe (and good CUC). However, some Syllids are known to eat sponges, tunicates, and - reportedly from some hobbyists - soft corals. The eating of soft corals is firmly disputed though, as many people (primarily from several years back, when Syllid worms were apparently more common hitchhikers) also report having them without issues even if the worms crawl over their soft corals regularly.
So, it’s probably safe/beneficial, but you can always sump it to be cautious, you could isolate it and offer it a cheap soft coral or two in addition to some fish food to see if it’s coral safe.
