those brain corals you have surrounding the colony might have gotten to it with their sweeper tentacles. happened to my green porcillopora except it got stung by my hammer coral. it was 3 inches away too. one night, i saw the hammer's sweeper tentacles all over the bottom half of my colony. next day, it looked like yours, but it got rtn. now i only have a small frag thats less then 1/2 inch. -_- gl with yours.