Corals Dying

In my 75 gallon i have about 15 different corals. Some are doing great (zoos, brain, mushrooms, hammer, etc) some arent doing to good (frogspawn, xenia) All of my levels and temperature are perfect. All my fish are healthy. My main concern is the frogspawn which was doing great is now losing color and slowly retreating into the frag. I know in an aquarium white on coral means death but my fish store said dying corals will break apart. I dont know if the oral is getting too much light since its high up on the rocks or if its not being fed enough but since i seen it dying i been over feeding it to bring coloration back. Any ideas why some corals arent too healthy??