I had the same issue. My first iPhone 6 had a terrible backlight bleeding where the clock is (top middle). I swapped it and the new one had these weird darker blotches on top (the most annoying one was near the battery). at one Apple Store they acknowledged the problem and wanted to replace the phone, but they didn't have mine (gold 64gb t-mobile) in stock so I went to another store where the dumb apple genius first tried to wipe the screen with a cloth and then started to argue with me that she doesn't see anything. Only after I told her that in Chicago they were willing to swap the phone did she agree to replace it... However she said it was the last time they were doing it, because they had to wait and see if it was a factory defect lol. I don't really care because I live in Europe anyway and I won't be back in the US for another couple of months. Anyway, the new iPhone had the same exact spots in the same place, but it seems that they have faded away, so maybe you should just hold on with replacing it and see if it fades away, too.