Ah, you young people... who don't remember the times when graphic cards used dedicated hardware overlays to display video, instead of this fancy textured video stuff nowadays. You'd get this effect all the time when moving around the mplayer window, except that the default Xv colorkey is blue, not green. It's impossible to fix this, it's just the nature of overlays.
Originally Posted by e8hffff
The way mplayer works around it with vdpau is it sets the colorkey to something that's almost black. Meaning it's effectively black to the human eye so you don't notice this effect, but it's not black to the computer so no "bleeding through".