I haven't added any new rock to this tank in over a year, so it has to be at least a year old.
What is this? I've never seen it before and I have no idea where it came from. It looks like a slug with a small flat shell on its back. I think I have seen them online and they are a good thing, but I wanted opinions so I can put it back in the tank.
(Oh, I did add a coral that was attached to a 2x3 rock...maybe it was on there?)