If you stare at this dot for a few moments it disappears. Stare at the bluish dot for awhile without moving your eyes or your head. The dot will gradually fade into the field of green.
Some people find that the dot fades more rapidly if they stare blankly at the edge of the square.
As soon as you move your head or eyes, notice that the blue reappears. Even though you're not aware of it, your eye is always making tiny jittery movements. Each time your eye moves it receives new information and sends it to your brain. You need this constant new information to see images. You may have noticed that although the dot fades just about everything else in your field of vision remains clear.