Question

why do copper vessels develop a greenish layer when exposed to moist air?

Answer 1

The copper of the articles slowly reacts with the moisture and the carbondioxide present in the air. So,it forms a layer of copper hydroxide and copper carbonate which is green in colour.

Answer 2

Objects containing copper corrode on exposure to air and moisture. They get coated with a green layer of basic copper carbonate CaCO3.Ca(OH)2