It's apparently automatically set on upgrade. Apple Accessibility Support walked me through disabling it, but it was their first time with this case and things may change.
1) For Screen Saver, choose one of the Other ones.
2) On the same settings pane, turn off Show on All Spaces
3) For Wallpaper, if it's not your preferred wallpaper then choose that or anything that doesn't have an icon on it. 

 As a side note, under Accessibility then Display, there's a new setting Auto-Play Animated Images which defaults to On, even if you have previously turned on Reduce Motion. 

 After doing the settings changes in the preceding toots of this thread, restarting the computer does not result in the motion screen.