Looks like something stung it almost- but tough to tell. If it just random started bleaching, it could just as easily continue to randomly RTN/Bleach. If it got stung- then that's sort of good news, should recover (a dip may not be a bad idea.) It might be a good idea to frag it once stabilized to remove any skeleton that doesn't have tissue on it- but if all that white is tissue that's just bleached, don't frag it obviously.
if it is dead, i would leave it for the new skeleton to grow over it...if is is bleached i would leave it as well, unless it gets worse, then frag as fast as you can