is called a colored afterimage. It is believed that it is due to
the long wave receptors in the eyes (responsible for seeing red) being fatigued
so you see green which is opposite color on the color wheel. The reason it
is in your peripheral vision is because there is something called surround that
allows us to perceive things we are not focused on.
If you continue to focus on the cross in the center of the image you will notice that the circle of violet circles will soon DISAPPEAR completely . . and you will see only the green spot (which is actually violet) . Check it out!
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Credits to: David Steele