Sorry. I don't own a Wii u myself yet. But I am very interested into the retroarch scene. I own a pi3 with retropie on it.
I read that on the retroarch tutorial:
"If you get a white screen when you start RetroArch, you're missing the assets package. If you can navigate the white menus, go to "Online Updater" and select "Download Assets". Otherwise, download the assets
here and extract it to sd:/retroarch/media. The next time you open RetroArch, all should be well."
Maybe it can help you.