I'm pretty sure [as I suspect you are] that it's not your ordinary bristleworm. Doesn't seem to be a eunicid. Leslie H may well spot this thread if it stays up long enough. She'd have a fair idea. Is any part of the body thicker than the other---[peanut worm]? Doesn't look to be. It's sure not a spaghetti worm.
I'd house this baby in the sump until ID'ed.
EDIT: ha! and there is the redoubtable Leslie H! Capitellid. New one on me.