This is a common optical phenomenon... or, rather, neurological... When we see a strong colour for a while and remove it, we tend to see a lingering smudge of the complementary colour in its place for a while, until our eye, or mind, adjusts to the new situation.
What we see is not a purple spot turning green, but the "hole" turning green. When we try to follow the hole we constantly move the eye so that we don't let our mind adjust to the situation, and thus we see no green spot. But, when we focus on the cross in the middle, we keep the eye still and then the phenomenon occur.