Copper? Or maybe brass?
Either way...
Copper turns green because of chemical reactions with the elements. ... Just as iron that is left unprotected in open air will corrode and form a flaky orange-red outer layer, copper that is exposed to the elements undergoes a series of chemical reactions that give the shiny metal a pale green outer layer called a patina.
Why Brass, Bronze and Copper Turn Green
But both are types of oxides. When you see that green layer on these metals (usually called patina or verdigris) it's because of a chemical reaction. The copper has reacted with oxygen, water, and carbon dioxide in the atmosphere.
Also
https://blog.amsoil.com/why-is-there...n-my-radiator/
Low-cost “green” coolants are the source of several problems, such as radiator sludge and scale deposits. For these reasons, most vehicle manufacturers have moved away from these coolants. And so should you. One solution is to formulate the coolant using organic-acid technology (OAT).