You have a Compositing level selected. I can see the "X". And as you ARE attempting to do some green screen I would think you have applied this and are now "midway" in setting up your CHROMA KEYING Fx process - Am I right . .or am I right . ..

First of remove the compositing X and check you can see your "actor" against the GScreen.

Been here done EXACTLY the same.

Now, I suggestion to all wanting to some support there? Do a screencast like our Chum? PLEASE!!!!

Grazie