Macbook Pro Display - Green Flicker only when Photoshop etc is open
Sounds like the beginnings of a GPU failure - there is an active repair program that runs through the end of the year for the 2011 MBP, so get your machine to the Apple Genius Bar and ask for a VST test.  Good luck! 

https://www.apple.com/support/macbookpro-videoissues/