Let's retrace:
* If the issue is on your phone display but doesn't appear on a screenshot it is caused by a stain on the display, such as sauce.
* If the issue appears on the phone display and screenshot it's a common problem caused by LCD damage or discoloration.
If it's the first issue grab paper towels and thoroughly clean off the display. If it's the second issue you have to replace the display screen either at home or at a repair shop. Just make sure your device still has warranty for qualified repair.
[guide|152975]