From what you said it sounds to me like White-band disease. Sometimes people call it white plague or white death (SORRY!) THere are two types. Type one causes a progressive white band or area that grows daily, usually by a few millimeters. Studies have been done, but no one is really sure about what the cause is (no abnormal levels of bacteria or fungi have been found) aside from stress. Type two is more virulent and results in a fast progression of necrosis. If the coral is bleaching before the tissue dies, then it's type two. It's usually not contagious to other corals. The only thing I can recommend for you is to A)first things first, make sure all your conditions are optimal. B)Remove the coral, put it in a QT, go to your dealer and ask him if he carries Lugol's solution (iodine in potassium iodide.) If you can find it, put one drop per twenty gallons in your quarantine tank. If things look REALLY bad, you may as well swab a little directly on the boundary between healthy and unhealthy tissue. The other option is to cut off the diseased part. You definitely want to do that in a QT. There's a product called liquid bandage that sometimes seems to help the coral heal...good luck...As a side note, you definitely want to make sure any specimins you buy look PERFECT! Live and learn...
