Just a thought, but if you have a new metal halide and you were running a less intense light you may have shocked your coral.

When we went from the PC light to our new metal halide things were not happy at first. we set the halide about 18 inches off the surface to start on only 4 hours a day. the corals closed up for almost a week then gradually started coming out and looking better.

we have since moved out halides down and are on for 6 hours of halide but it took a while for the corals to adjust.

I dont know if this is causing you issues but you may want to think about the timing of your issues in the tank.