my wife showed me this on her S5 and i thought her screen was "broken." i tried it on mine, and it does the exact same thing. check out the 7th and 8th images in the link below, and scroll the photos up and down. what do you see? do the images "bleed" underneath the rock in the center of the image?
it's most noticeable on a screen with the brightness turned down and in a dark room:
Crazy Climbers - Imgur