I have this in my 75g tank. I got this along with some other live rock recently purchased from an LFS. I really thought it was a dead oyster or something...and that corals had grown on its back. There were a plethora of small feather dusters, so I thought it would be cool in my tank. ...until it unhinged itself in my hand. That was a bit unexpected to say the least.
Let me know what you think. I'm leaning toward abalone...or alien. Overall, it is about the size of my fist -- that includes the coral.