What Gary said. I had a similar problem, turned out my Alk was too high and Ca and Mg were too low. After getting these parameters back where they should be, I immediatley saw an improvement in all my corals.

Also check your water temperature, especially after your lights have been on for a couple of hours. High temperature can cause the corals to bleach and recede. I had this happen when my fans failed to come on. Luckily I caught it before everything died and eventually everything came back.