Extra small dot on the left corner screen

Does this dot appear on All dark backgrounds in any app, or just on your Lock Screen? I'm wondering if it might be part of the wallpaper you're using for your lock screen, especially since it is a space photo and could be a star in the wallpaper.

It looks like a stuck pixel.

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple