This is a glass anemone, similar to aptasia but they are swim (sometimes called swimming anemone). Copper banded butterfly fish will eat them, but you do not want to let these guys get started in your tank. Nasty! Aquatek used to have them in their front coral tank and they got major out of hand. They added 4 copper banded butterfly's which reduced the population, but they never completely got rid of them till they emptied the tank.