And no, the Moon Terminator is nothing to do with Arnold. The “terminator” in this case is the line between the lit and dark sides of the moon. During those times when the moon and the sun are in the sky together, there’s something odd about it. The line defining the lit side of the moon doesn’t line up with the sun, the thing that’s lighting it.
This video from YouTuber Vsauce attempts to explain the phenomenon of the Moon Terminator illusion. A camera is used to explain some of the various principles involved. It’s well worth watching, as it also goes some way towards also explaining some of the issues photographers hit regularly. Understanding the causes of them can help us to resolve them.