add these two lines under [SeatDefaults] in /etc/lightdm/lightdm.conf.d/90.nvidia.conf
greeter-hide-users=true
greeter-show-manual-login=true
new way:
create /etc/lightdm/lightdm.conf.d/10-ubuntu.conf
with:
[Seat:*]
user-session=ubuntu
greeter-show-manual-login=true
greeter-hide-users=true
allow-guest=false