I recently encountered an issue with the Deepin lightdm greeter that I'd like to elaborate an answer on, as finding it is a little obscure and this would, hopefully, help anyone searching around.
The same issue is found on this post. I'd comment, but I don't have 50 rep!
When using the proprietary Nvidia drivers, you may come across an issue where your login screen / lock screen come up as black or white, maybe with an icon. This occurred for me with the deepin lightdm greeter. As an initial workaround, I switched over to the gnome greeter.