That’s patina. And people pay EXTRA for it. Go online, they even sell Patina paint!!! Haha
Is it algae or just aging patina?
You can wipe it off or hose it off. William has your answer! Will reappear periodically if statue is in a humid location with little to no sunlight.
Barbara, Purchase bronze cleaner, and then use some car wax to seal out the elements. Aloha!
I would use a Mr. Clean Magic Eraser, followed by rinsing with the hose.