What's this little white circle on my Mac's screen? Can anyone help me identify this little circle on the screen of my Mac?
The best way to catch the culprit is to quit running apps (both in Dock and Menu bar) one by one until the circle goes away.

As identified by OP by following the above mentioned approach, the circle is created by an app called MacID which along with along with it's companion iOS app Unlox lets one unlock their Mac using their iOS device.