It depends on the material in the nose pad. It sounds the metal in your nose pads is like bronze or copper. Sweat and oil deposits on the nose make your nose moist. Then the bronze or corpper will get oxidation in the moist place and then they turn green.
If your frame is mental, it can be caused by oxidation. Most mental frames contain bronze, the air and sweats make it oxidize and finally turn green. You should keep your glasses dry especially when you sweat a lot.