a couple weeks ago I purchased a purple short tentacle plate coral. everything seemed fine, it had good color, it was eating and its polyps were extended. then it retracted it polyps, started showing signs of white around its edges and its mouth stayed open. it is now starting to turn brown and more white is showing. I checked water paramaters and all are good except calcium is a little high. It is on the substrate of tahitian moon sand, and is not eating. by fish, crocea clam and xenia are all doing well. Can anyone help?