Oh i forgot to post my inhabitants, i dont have much right now since the tank is fairly new, i've got 8 snails, 1 emerald crab, 1 fireshrimp, a few bristleworms and a couple of random hitchhikers like little worm dudes, also i have some corals, a pink hammerhead, some spotted mushrooms and some candy canes
yeah and now its gone and i havnt noticed any weird outbreaks of stuff, well i did all of a sudden get a nano starfish outbreak which is really cool but i doubt that they came from that sac. i'm guessing it was just a weird build up of algae
Looks like a dying something. like a lone zooanthid or something that was hitchhiking. Probably a good thing, might have been an atapsia anemone. As for the nano stars, if they get to be too much, get a harlequin shrimp. They love those little stars, but then again, the fireshrimp mite take him out!