I had been using Gnome on Ubuntu 16.04 instead of the usual Unity. When I updated to 18.04, this weird double u's logo persisted through the upgrade and now both of my home and lock screens are different.
That's GNOME. It's a little different from what you're used to in 16.04. What you're seeing is the GNOME splash (boot/welcome) screen, and gdm3 login screen.
ls -al /etc/alternatives/*plymouth* # shows what the splash screen is currently set to.
You can change it back to the old-school Ubuntu splash screens using the
update-alternatives command. See
man update-alternatives for help.
If you're running with gdm3 display manager, and you wish to run lightdm instead, you can
sudo dpkg-reconfigure lightdm.
But... I'd recommend leaving this all alone, and just take some time to get used to the new look.