I work at an LFS and these showed up in the frag table several months ago. I checked the hitchhiker thread and didn't see them. They are slimy and squishy like an anemone or coral, and they retract somewhat when harassed or when a shadow passes over. There's There's a few that I've kind of tried to scrape off with my nail, but they seem fine after a day or two. They are fixed in place and do not move. They kind of look like individual clustered flower heads. They don't seem to be multiplying very quickly, don't seem to bother anything, and I've not seen any of the fish or CUC picking at them. They're in a few places around the table, but they all seem to be in the opposite half of the table away from the wavemaker, if that's relevant.