You can try changing what CIOS is used to load the game. By default it used whats installed in slot 249. Depending on what guide you followed to hack your Wii you may have additional CIOSes installed in slots 250 and 251.
When you click on the game in USB Loader GX click on the settings button. I don't remember exactly where the option is but you should be able to find somewhere an option (game load maybe?) that will allow you to change what slot to use. Try typing in 250 and try loading it up. If that doesn't work, do the same thing but this time try 251. If that still doesn't work then run a syscheck on your Wii using this guide here (
https://gbatemp.net/threads/posting-a-system-check.300279/) and post the results here. This will let me know if you have the right CIOSes installed and if not I can guide you on how to update.
Edit: Just checked the compatibility list here (
http://wiki.gbatemp.net/wiki/USB_Game_Compatibility_Table#C) and it says that it works with base 57 (which is usually installed in slot 250 depending on the guide you followed). So give that a try.