Can't tell what that is... but I killed a very large plate 14" in diameter. My wife said leave it in the tank - I thought as punishment... But she'd read somewhere when they are dying they try to breed. She was right!
We now have 6 tiny plate corals - 4 a bit larger than a pencil eraser and 2 the size of #2 pencil lead. She finally allowed me to cut the skeleton into pieces so we can better watch them grow.