I had installed Xubuntu in the past. Now the "mouse" screen showed up after updating to 19.04. After researching I have tried many "solutions" and nothing works. It is very frustrating. I set the lock screen in the background settings app and it does not phase it. I deleted the Plymouth themes that were not GNOME or Ubuntu and that didn't help.
This may be the "greeter" screen that comes up after everything loads and you are to pick the user.
Someone said Nautilus takes over this screen. I am not sure what to do at this point. Help in understanding this would be appreciated.