Being that the discharge is green, that's a pretty good sign of infection.
GSDs, particularly senior GSDs are suceptible to infections. I do not know how to counter this really. As a breed they tend to run low IgA. Many of them are also sensitive to immune boosters too. I did recently read that GABA, used for anxiety, can increase IgA, as well as melatonin. It's not like you can have your dog ue a netti pot to keep that nose cleaned out.