I get that when i REALLY concentrate. It depends on how mild it is.
is the green/yellow thing obviously strong, or pretty transparent?
If it's transparant-ish and not really noticable, your concentrating too hard. It's like screen-burn on a TV when you leave it paused on something for a long time.
If it's pretty strong and way too obvious, you might want to see a doctor about it
EDIT: I'm only saying to see a doctor because s/he may come up with a better answer than us, because s/he probably knows more about this than I do.
Personally, I only get the transparant-ish one, so mine has only been diagnosed as the screen-burn kind of thing. Still, yours might still be this sort of thing too, it just depends on how you describe your "transparant" to mine.
EDIT: Since you've mentioned that you've got perfect eyesight, its probably what I... and everyone else has.
I reccommend that you try not to concentrate too hard on something / someone if it bothers you, or move your eyes a little every now and then.