I didn’t get that when I tried it, but likely because the configuration is different since my machine was updated from older versions that used different greeters. I’m guessing it installed lightdm-gtk-greeter as a dependency and set that up. You maybe be able to reconfigure lightdm
sudo dpkg-reconfigure lightdm
or you might need to remove lightdm-gtk-greeter or reinstall slick greeter.
For the missing fonts, my first guess is that they’re installed in your user directory. They need to be installed system-wide for the greeter to see them.