Monti Cap

I was given a couple of frags of Montipora. I placed them in my tank toward the top. They seemed to be doing good at first and they began to encrust on the rock they were placed on. But now, after a few weeks, the edges are turning white. They were slightly white when I got them home but the color seemed good and the white appeared to be going away. Now the white area is growing around the edges. Could my lights be too intense? I have two AI Hydras on a 120 gallon tank. All other corals are doing good but they are all LPS and the Monti is my only SPS.