That is because a Samsung uses OLED which suffers burn in (since blacks emit no light on these screens). Newer phones do subtle shifting of long running pixels to avoid burn in. You can “fix it” by putting a light gray image as full screen for an hour or two a night. This will eventually ruin your phone... my Samsung s2 and note 4 which I use as a on-the-go baby monitor are almost useless for reading any kind of text