Copper, when in contact with the skin, forms chelates with human sweat (sometimes seen as green deposits) and is thus absorbed through skin." "Copper chelates are more effective and less toxic than...
It may not seem so but copper is reactive, when in contact with oxygen, carbon dioxide and sulfur dioxide, it turns respectively into: In relatively clean atmosphere without sulfur dioxide - copper...
The substance would be... the copper itself! But it's not that trivial. The green substance is the product of the corrosion of copper and its alloys. In clean atmosphere product is called copper (II)...