Does this 'fixed grid' only appear when some transparency effect is going on (like smoke, fog...)? Because then i think its normal, happens on my WiiU + 4K TV too!
Should be the so called 'Dithering'. Apparently most Wii games are only 16bit color depth and the digital signal + barebone upscaling of the WiiU makes those imperfections (that you wouldn't notice on a CRT + analog composite signal back in the days on an original Wii) more pronounced then ever.