Yeah that's definitely it. Did a search for led multiplexing and ghost pixels and I came across this:
Improperly designed LED-multiplexing circuits can create ghost images. Ghost images result when parasitic current flows through LEDs that are intended to be in an off state (i.e. no current flowing through them); this causes very faint illumination or ghosting. These ghost-image currents typically result from the discharging of stray capacitances associated with the large, common-LED anode-node tracks and the slightly forward-biased LEDs themselves.
Faint ghosting images from parasitic currents can occur when the multiplexing changes phases from MUX0-bar to MUX1-bar and vice versa. The effect is most pronounced when the LEDs on the multiplexed circuits are different colors (light wavelengths) and, hence, has significantly different voltage drops for a given current flow.
The ghost-image currents can be eliminated by providing a discharge path for the parasitic node capacitances and providing a time for the discharge to occur.
So I'm guessing the fix (if that really is a fix) would need to be implemented in the manufacturing stage of the LED matrix or the shield? Well, it's not a huge annoyance and it only appears on certain images when there is a black background. Something I can work around.