In my experience a high pH can cause bleaching. If you are adding buffers you maybe overdosing. There are some other suggestions in this article that may be of some help.
http://www.advancedaquarist.com/issu...e2002/chem.htm
Also, I have never heard of a yellow colt coral. Always thought that they were brown or tan. Is it possible that it was dyed and that the animal is suffering for it?
Larry