is that green square white? (or close to?) i'm leaning towards a game board issue. or the video wires are shorting together (checked for wires touching together from the game board up to the monitor.)
if this was a monitor issue, it would cause the screen to wash out with red, green, or blue... or the red,green, or blue squares would go black. this isn't happening here.
I don't know anything about your particular board. but sometimes using a pencil eraser on the contacts on the edge connector of the game board can clear up all sorts of issues.