Login screen on macOS Sierra suddenly with gray background yesterday after waking my Macbook Air (Early 2015) with macOS Sierra from sleep I was greeted with gray login screen. Now I have gray every time I wake up my Mac.

First time I had the same color as background on dock for some reason but that went away after I disabled "Reduce transparency" in Accessibility settings.

I did some googling and found that this may be related to FileVault (since everything is encrypted OS presents gray background) however I had FileVault turned on since I got my Macbook and never encountered this gray thing.

What is the cause? My main worry is that it might indicate some problems with my computer. Thanks.
When your machine restarts with FileVault enabled, the gray screen login is to unlock FileVault. There are no problems with your machine.