It is a teeny tiny little tube worm, but it's alive! I was staring at the live rock--the murk is slowly settling, visibility is now about two inches--trying to figure out what the white tubes, like very narrow wiggly pasta, attached to the rock were, and then one poked its little head out! It's like a tiny little feather duster, red and white striped, maybe an eighth of an inch long.
There's a whole bunch of the little tubes. I have no idea how many of them will have survived the transit on the live rock, or will survive cycling the tank, but eeee!
The other thing I'm realizing--as I google frantically trying to ID everything--is that this is either the best hobby ever or the most terrifying. The number of stories that come up with people having something sitting on a rock that's apparently unknown to science, where the experts all go "Yeah, no clue what that is! Good luck!" or "It could be one of twenty thousand related species, so just leave it alone, it's probably harmless," or occasionally "I think a friend of mine had that. No, he never identified it either. Occasionally it would get off the rock and roam the tank at night to feed, but then it reattached itself in the morning," is really a little disturbing.
Possibly it's because, even if what you get is happy tank-raised rock like mine, you're still at the end of a chain that ultimately started somewhere in the ocean, in one of the most diverse ecosystems on earth, and there could be anything there.
Probably all I'll get is weird teeny worms--or if I'm unlucky, pest anenomes, and if I was hellishly unlucky, mantis shrimp--but still! I have six gallons of the unknown!
Freshwater was never this fun.
ETA: At least four currently live tube worms of that variety, and one rather larger one that's got a solid red fan maybe a little under a quarter of an inch long. His tube is practically embedded in the chunk of rock.
God, I can't remember the last time I was this excited over something without a backbone!
FETA: This is what the red one looks like--he's only about a half inch long, so he's not as impressive as this photo (I'd need one helluva macro lens to get him) but he's very pretty in a very small way.