too much light doesn't make sense judging from your picture. it's only half the coral. is the white part spreading fast? it looks like RTN to me (rapid tissue necrosis), and there are multiple causes of this.
if it's RTN, then i would frag off the good part right now. make sure you leave some good flesh with the RTN, so that none is remaining on the living frag.
some of the causes include, but are not limited to:
chemical issues (stings/poisoned by neighboring corals)