If this is a new tank that you've set up with coraliney liverock it's pretty common to lose a fair amount of it. Differences in stability, parameters, and light will make it unhappy. In a while if you keep things on track it will come back. Besides the particular disrupted colonies, there's also a few diff species on the rocks. So when it is settled in they have found their happy spots and when things get moved they all have to populate according to their new various placements.