you're carbonate hardness is pretty high, but I'm not sure that would do this. How long did you give the corallaling between changing the LEDs? Was it well established in your tank and suddenly turned white, or is it new rock? The increased light may bleach out coralline just the same as it does to coral which also needs aclimated to light changes. If it's new rock, it could be light shock, you'd be able to tell because the darker areas would be less affected, like the underside of the rock. Often people report corraline grows better on the undersids of rocks especialy if they have very bright lights.
What I'd do at this point is back off of the lighting a bit for a few weeks, and double check your kH levels as 12 is quite high. That may be causing a problem, but I suspect the lighting for this issue.
