Try turning the phone off and then back on and see if that resolves it. If it continues, then try starting it up in "Safe Mode" and see if it happens in there. If it doesn't, then you likely have something on the phone that is interfering. Virus, malware, bad app, etc., that will have to be removed. If, however, it happens in "Safe Mode" as well, then you will likely have to have the device looked at for display problems. If it is under warranty still then you would contact the manufacturer and if not then a local tech.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.