A Malachite green layer is formed on the copper surface due to corrosion.
Malachite is formed due to the weathering of the copper ores. It is copper carbonate hydroxide mineral.
The copper when exposed to moisture for a long time forms a green layer on its surface. This is due to the oxidation of copper.
The oxidation of Cu form copper oxide, which then reacts with carbon dioxide to form copper carbonate, and this gives the surface of copper a green colour.