aerugo
/əˈruːɡəʊ/noun
The patina that copper and brass acquire with age; verdigris. Occasionally more widely: an analogous coating on any metal.
Origin
Late Middle English (in an earlier sense). From classical Latin aerūgō verdigris, in post-classical Latin also mildew of plants from aer-, aes brass + -ūgō, suffix forming nouns.