I'm betting it's the lights, especially if the die off is affecting the top of the tank more than the bottom, and the coraline is turning whiter or paler in color. Some coralines like lights, some don't.
You should check your calcium and alk, just to be sure.
If it is your lights, you should eventually see other types of coraline growing in the high light spots. You can speed this by adding a coraline booster pack which will introduce some new coraline species into your tank. They are available on a lot of different web sites, and are just chips of live rock.