Indigenous

https://en.wikipedia.org/wiki/Bronze_disease

Bronze disease is an irreversible and nearly inexorable corrosion process that occurs when chlorides come into contact with bronze or other copper-bearing alloys.[1] It can occur as both a dark green coating, or as a much lighter whitish fuzzy or furry green coating.[1] It is not a bacterial infection, but the result of a chemical reaction with the chlorides that usually occurs due to contamination of the bronze object by saltwater or from burial in specific types of soil where chloride salts are present.[1] If not treated, complete destruction of the affected artifact is possible.[1] Treatment is very difficult, costly and not always effective.

This statue already seems to have it on the legs from de-icing salts or something. It'd be a shame if you started spraying super salty water on its face every time you walk past.