That's a bad one. It's Ophiarachna incrassata, also known as the Green Death Star" Here's what Ron Shimek had to say about it in his book "Marine Invertebrates":
"...has been documented to eat firefishes, damselfishes, mandarinfishes, blennies, small gobies, cleaner wrasses, cleaner shrimps, and crabs."
It creeps up on sleeping fish at night, or stands with the body elevated & drops down on anything foolish enough to move under it.