Might be a variety of worm that eats them, but those arent real common. My thought would be to move the GSP somewhere with better waterflow. Im guessing debris is settling down between the polyps, which will irritate the coral enough to keep it closed up, and would be a great food source fo bristle worms at night. Just keep it there for a while and see how it does.. hopefully it will recover. They can stay closed for months sometimes and look like hell, then start to rebound.