Probably a bad pixel. Turn off your laptop, then apply pressure on your screen where the dot was. Turn on the laptop while still holding it for a few seconds.
CAUTION:
Do not apply too much pressure or you might break the screen altogether.
This is a ‘‘trick’’ and by no means is it guarranteed to fix your problem.
Most of the time people change their screen completely when they have bad or dead pixels.
I have learned to live with mine ;) (single dead pixel right in the middle of the screen)