I have recently had the same problem with my Orange Montipora. I removed some pieces of rock to clean them with a power wash of tap water three days ago, including the two corals below (Orange Monti on top, unknown coral below). The rocks had algae and aiptasia on them, surrounding the corals, and that I thought that may be inhibiting the corals growth. So I power washed the rock around them, trying to shield the corals from the fresh water, but when returning these two pieces to the tank, the Orange Monti (top coral) was completely bleached. I was shocked . The other coral (unknown coral) didn't seem bothered at all.
Now the Orange Monti (top coral) has orange polyps while much of the coral is still white. No algae has grown yet...
Poor picture
What type of coral is the bottom coral?