Try scraping the green stuff off with your fingernail or a wooden pick, if it comes off it is probably bronze disease but if it doesn’t it is probably verdigris which actually protects it instead of destroying it. Here is the wikipedia article which lists a number of ways to protect coins from bronze disease and remove bronze disease (not completely though). https://en.wikipedia.org/wiki/Bronze_disease
I tried scraping gently with a fingernail and nothing came off so its probably verdigiris. Also I think it's a bit too dark to be bronze disease but I'm not an expert.