Green screen on wake-up

I have a 3 GHz Intel Core i7 Mac Mini (late 2014) with 16 gigs memory and a 256 gig SSD. The system is El Capitan with the latest updates.

For the last couple of months my monitor screen will be a bright green color whenever I wake the Mini from a sleep. Sometimes the log-in dialog is invisible in the green mess, but I just type in my password blind and get logged in that way.

It's functions perfectly after I log in, and is perfectly normal (no green) when I cold boot. The monitor is an LG Ultrawide 34" and I'm using the HDMI connection. Any help is appreciated