Kind of looks like an asterina starfish with most of the legs having been chewed off and a new set is developing? I've seen asterina that look lopsided and this "worm" apppears to have starfish feet, at least to my old eyes.
Asterina starfish that lost a few legs. It will grow them back though. They are not too bad for the tank but can multiply like crazy and eventually become a small problem
Yeah, as mentioned, that's an "Asterina" starfish (technically Aquilonastra - Asterina is a different genus in the Asterinidae family). They reproduce by splitting (and/or dropping legs) and having both halves (or the dropped legs) regrow into their own full starfish, so seeing them like this isn't too uncommon.