Fretwire is normally "nickel silver," which is a white brass, always consisting primarily of copper, with a fair amount of nickel, and usually zinc. I have found the most commonly available alloy to be C770, which is .55 Cu, .27 Zn, .18 Ni, with traces of Fe, Mn, and Pb. Most fretwire is stated as 18% nickel, and so quite possibly is C770. 12% nickel fretwire is also available, and is possibly another common alloy, C792, which is .615 Cu, .255 Zn, .12 Ni, .01 Pb. Hence I would think the the most likely cause of green corrosion found on frets would be the copper.