Your manually installed driver is probably causing the problem. The fix will be to uninstall it, reboot which should pick up the nouveau driver, and then the Software & Updates screen you show should offer the Nvidia 375.66 as an active choice.
Locate any line like "blacklist nouveau" in any file in /etc/modprobe.d These should all disappear when you uninstall manually your proprietary driver. Uninstall the driver. Check that no "blacklist nouveau" line is left, if so, delete it or comment it out.
Reboot, and you should have the nouveau driver in use:
sudo lshw -C video
Now use the Software & Updater to install the Nvidia 375.66, which fixes some artifact problems with earlier 375 drivers. Reboot and you should be using the nvidia driver (repeat above check).