I had this happen on the replacement Moto X that I currently have. It happened once and has not happened since. I also rebooted my phone to make it go away. I hope this isn't a sign of bad hardware.
I've had that screen a few times also. I have never dropped this phone either. Usually happens when I pull it out of my pocket. Screen still works normally but can't get it back to normal without a reboot. Must be a software glitch. I did a factory reset after upgrading to marshmallow. It only happens once a month or so.
For the record, I did a factory reset, mostly because of an old issue I had, described on one of my other posts here.
After the factory reset the "screen on/off while talking" seems to be fixed.