Seeing black spots on display usually indicates damage to the LCD or OLED panel inside the phone. This may occur in the event that the device has been dropped, over pressed or left in very hot or cold weather. The spot might be small but at times grows with time as the internal pixels malfunction. Sadly, this issue can hardly be addressed by software solutions since it is connected to the physical screen elements. Rebooting the computer will be a good step in eliminating a temporary glitch but in case the spot remains, the surest bet is to change the display panel.
