The macOS Desktop background changing will not be EFI related. The OC or Clover EFI will deal with and control boot options/settings, not macOS settings such as the desktop background.
If you use System Preferences > Desktop & ScreenSaver pane to set the desktop background you need to make sure you have not inadvertently ticked the change picture option, located at the bottom of the Desktop pane. As this would automatically change the background after a set period of time, or upon a reboot of the system.
System Preferences > Desktop Pane
The Change picture options are highlighted at the bottom of the pane in red rectangle
If that is not causing the changes you may need to use the ClearNvram or ResetNvramEntry.efi option from your boot loaders boot screen.
In Clover this is executed by pressing the F11 key while the system shows the Clover boot screen and then manually rebooting the system, so all the existing/old NVRAM entries are removed.
In OpenCore this is executed by pressing the spacebar while the system shows the boot screen, and then selecting ResetNvramEntry.efi from the Drivers/Tools that are unhidden. Using ResetNvramEntry.efi will automatically reboot the system.
Using ClearNvram.efi will not automatically reboot the system. If you use ClearNvram.efi Tool you need to use the ResetSystem.efi tool so the old Nvram entries are removed before you boot into macOS.