I had a few of these in the back corner of my tank until I figured out they eat fish! mine were a little bigger than yours but they swallowed fairy wrasses, sifter gobies, and any other smaller fish trying to hide behind the rocks. I used to try to bury them with rubble and rip them up with a coathanger but no luck. I had to take a 3rd of my tank down to remove them! They were pink tipped, pretty nice looking pest. I gave them to Hector. they are actually a shroom(Pseudocorynactis) and can move. Some large ones go for good $. Tidal gardens in copley had some.