A Strange Guest User at login screen. Even though, I turned off the login for Guest User in System Preferences.
I worried about my Macbook's Login User in High Sierra. In System Preferences, I have turned off "Guest" user and definitely only me as the administrator with Password for login. But when I turned off my Macbook and turned it on again, I found two users that have shown on the login screen those are My Account with my Image and Guest User with a strange Background Image (not default as grey/white). I suspected that is a Stalker/Cracker? or Probably High Sierra still has some bugs. This case comes when I every shut down and turn it on my MacBook again.
MacBook Pro (Retina, 13-inch,Early 2015), macOS High Sierra (10.13), have installed Supplemental Update