First of all… 
DO NOT USE grub-customizer IN MANJARO!
Seriously, do not use it. Manjaro uses a customized version of GRUB, and grub-customizer completely messes that up.
Now, in order to fix your problem, you need to uninstall grub-customizer and clean out the mess it made, and then reinstall GRUB. It is important that you do not shut down or reboot your system before all of this is done…
sudo pacman -R grub-customizer
sudo rm -f /etc/grub.d/*
sudo pacman -S grub
sudo grub-install --recheck --no-rs-codes
sudo update-grub
Cross your fingers… 