I agree with trigger40 that this could indicate stress, but your water params sound good. Have any new fish been added to the system that might be having a territorial issue with the angel. Also, be aware that when the large angels change color in captivity they generally do not develop the beautiful coloration normally seen in the ocean, but are frequently pale. This might be what you are seeing with this fish. Continue to give it a varied diet with whatever sponge you can find and perhaps add a marine vitamin mix. Good luck!