If you use the Steam version, you should try verifying the file integrity option. If you used the X-Plane installer to get the sim, use the installer again and pick the "update" option. These options will scan the install and repair any missing or modified default files.
If that still doesn't help, I would recommend removing the add on next. I do not have a moon.png file at that location in my clean, default install of X-Plane but it is still working.