Found a new hitchhiker the other day that I believe I've ID'd as Elysia coodgeensis although I haven't been able to get a good picture to get second opinions on this. It appears that this is probably harmless, I haven't added any rock or anything to the tank for a few months, and nothing has come up missing or died unexpectedly. When I first noticed him though, he was sticking out of a rock a bit and emitting puffs of a white fluid. It wasn't think and milky and seemed to dissolve in the water column quickly. Not knowing what it was doing, I tried to capture the critter, but was unable to. I saw him again this morning on the rockwork but he moved pretty quickly once the lights came on. Should I be concerned?