I couldn’t match it to any of the worms in the common critter sticky (i.e. did not appear to be a bristle, fireworm, or bobbit), but I have zero experience identifying worms. Set up my tank by starting with live rock and sand from the gulf (Tampa Bay Saltwater) about a month ago. This is the first I’ve seen of this worm this morning when the lights came on, so I think it is nocturnal. What do you think it is? Thanks!