It's most likely nerve damage or bruising. This happens occasionally to things like discus. Sometimes they heal and the coloration goes back to normal.
Thanks for the information. This is the first time I've seen this & will be keeping an eye on her.I have heard it is attributed to nerve damage, but haven't heard it being due to bruising. I had it happen on a wild caught angel. The fish was otherwise fine in appearance, though it died suddenly a few months later. Not sure what the cause was.