[pogodzib]

It could very well be caused by several issues pointed out.

Since your lighting is very strong, you need to be careful to acclimate your corals slowly or you'll bleach the corals. How do you acclimate them? You should either screen the tank or raise your lights, along with shortening your photo period and gradually remove some screens or lower your lights.

Phosphate removers can shock corals and cause them to bleach, so if you don't have an algae problem or high phosphates, try running minimal/no phosphate absorbing media.