Don't have much to offer. The bright green oxidation is called verdigris. Where it is there is no metal. Leave it alone. No known cure aside from time travel.
Now think of the sun just before sunset, the long shadows which are cast. Or if you lose something small in a room. Myself, I turn off the lights, kneel down with the side of my head near the floor and a minimag light in front of me. As I scan the floor with the light, the item is highlighted against a dark background. Much easier to find small things this way.
To apply this to your button might show a few additional letters. You don't, obviously, have to kneel on the floor. Simply light the button from a shallow angle. Hope this idea helps a little.