/etc/lightdm/lightdm.conf:autologin-user=norm
/etc/lightdm/lightdm.conf:autologin-user-timeout=10
norm@norm-MacBookAir:~$ grep -r autologin-user /usr/share/lightdm/lightdm.conf.d
norm@norm-MacBookAir:~$ grep -r autologin-user /usr/share/lightdm/lightdm.conf.d
norm@norm-MacBookAir:~$ lightdm --show-config
   [Seat:*]
E  guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
J  allow-guest=true
D  greeter-wrapper=/usr/lib/lightdm/lightdm-greeter-session
F  greeter-session=slick-greeter
G  user-session=mate
H  xserver-command=X -core
I  type=xlocal
I  display-setup-script=/sbin/prime-offload
I  display-stopped-script=/sbin/prime-switch
J  greeter-allow-guest=true
L  greeter-show-manual-login=true
L  autologin-guest=false
L  autologin-user=norm
L  autologin-user-timeout=10
   [LightDM]
C  backup-logs=false
J  guest-account-script=arctica-greeter-guest-account-script
Sources:
A  /usr/share/lightdm/lightdm.conf.d/50-arctica-greeter-guest-wrapper.conf
B  /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
C  /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
D  /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
E  /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
F  /usr/share/lightdm/lightdm.conf.d/50-slick-greeter.conf
G  /usr/share/lightdm/lightdm.conf.d/50-ubuntu-mate.conf
H  /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
I  /usr/share/lightdm/lightdm.conf.d/90-nvidia.conf
J  /etc/lightdm/lightdm.conf.d/91-arctica-greeter-guest-session.conf
K  /etc/lightdm/lightdm.conf.d/91-arctica-greeter-mate.conf
L  /etc/lightdm/lightdm.conf
Your lightdm is configured to autologin user norm.
This line is coming from file:
So, edit your /etc/lightdm/lightdm.conf and remove the corresponding line.
OR
Since you are using slick-greeter now:
you can open your Login Window GUI and disable autologin there by clearing the username field.
              
              
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          All is now set!
I thank all in this thread who assisted.
              
              
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