This is what Boris tech support emailed me:
The green image is usually due to the RT settings within the Final Cut
Preferences; sometimes they default to ‘Unlimited’, but it’s better to
operate in ‘Safe’ mode. To check/change this, go under the Final Cut Pro
menu, select System Settings, and go into the ‘Playback Control’ tab.
Inside there, the first pulldown should be RT – if it’s set for Unlimited,
change it to Safe. If it’s set to Safe, try setting it to Unlimited, and
then back to Safe.
This works, as it solves the green screen but kind of drag to run ion Safe mode.
Hope this helps