New to this place first post. similar problem occurred last night with a little more detail (photo will be attached) look to left of Apps icon. I have a set of matching phones, (s3's) one was pristine[A] as i had received it from a person only after 1 month of use and stored it for the future. the other my original purchased when the model came out.
having done a backup of old with Kies3 and a reload to new]A] transferring only the basics were allowed to. I now decided to use NFC/ S Beam to transfer all those important desktop items.
It was during this process that suddenly on both phones [A&B] the white spot appeared. [A] was plugged into power supply. Both appear in the same spot.
Powering on/off makes no change. Tried a pixel checker with no indication of a burnout or damage.