Usually a lack of light will result in browning, not bleaching.
That looks like it was shocked recently which can result in rapid tissue necrosis. This can trigger the same thing in other SPS
corals. The shock/stress may have been from a water change where something didn't match the chemistry already in the tank (pH, temp, salinity, etc.). I would probably frag it about 1/2" above the most advanced bleaching in a hope of saving at least some of this coral.
How long has it been going on?