The nose pieces turn green because of the oxidizing reaction
of bronze with sweat and natural skin oil. To prevent it from turning green, you should keep your frame dry. When you have sweats, clean it timely with a piece of cloth. Remove the grime and dirt on a regular basis. If you fail to prevent or reduce discoloration, you can replace your current nose pads with a fresh pair.
Main cause of the problem is human sweat. I also wear glasses, this situation appears more in the summer. I have tried a variety of methods, but the problem cannot eradicate. On top of paying attention to daily maintenance, the best way is to go to optical shop to change it regularly; it's very cheap and free in some specialty store.