Kmando -
Nickel plating has a tendency to tarnish due to the oil & sweat on your hands. Other parts will tarnish when exposed to the air (salt air and humidity). The plating can be polished with a simi-chrome polish, which is also used on motorcycle parts. You have to be careful not to over use the polish, as it will remove the nickel finish all together. It sounds like the coating you see is tarnish. After awhile, the tarnish is difficult to remove even with the polish.
I would have to see the banjo or a photo to advise you what the issue is with the banjo and what can be done with it.
