I just saw this dude crawling and I was thinking it was a MASSIVE bristle worm but upon further inspection there’s no bristles and not sure if it’s safe to keep, it had little legs like a millepede unless it’s shed the bristles off or something?? Not sure but it had legs what what I see
This is Oenone fulgida, a predatory clam worm and pest in the marine aquarium. They will consume your mollusks, including clams, snails, etc. by enveloping them in toxic slime.