Copper Oxide? Help me get rid of this.

[TheMash]

I cleaned one of the two "brown" fittings. It isnt brown now, but its dark grey/black, like zinc I think, or maybe the classic Copper Oxide.
You should see a pinky stain, in typical color of copper, I think it will not hurt,will it?






I used some abrasive paper on the dark grey (or black, depends on how you consider it), the result is the nickel plating come again out.
So the brown, and then grey, layer was oxide of something else, its not oxide of the fitting itself I think.
Maybe its copper oxide?