I have been unsuccessfully trying to identify this creature. He came to us in a piece of live rock from our LFS. We love him. Just wish I knew what he was. During the day, with all lights on, he is all puffed up like in the photo. Currently about 1.5" in diameter and about a 1" in height. In full darkness he flattens out, and then you see tentacles and a center mouth. We do feed him pieces of shrimp, at which time he flattens out a bit so his tentacle appear to grab the food. At times the "puffy" body mass will also become flat against the rock with no tentacles showing.