That's zooxanthellae.
If you added new lights that could be bleaching. Same thing if you just introduced them into the tank. Light shock could provoke that.
High temperatures are also known to lead to that, again as bleaching.
If you didn't change your lights, or if you didn't just introduced the polyps to your tank, you don't have to worry. Those PEs do that from time to time. Make sure the temp isn't too high!
Grandis.