The medal is likely made of bronze, which is an alloy primarily composed of copper (Cu) with small amounts of other metals like tin (Sn) or zinc (Zn).
The greenish layer that formed on the surface of the bronze medal is known as "patina." Patina is a thin layer that develops on the surface of metals like copper and bronze over time due to chemical reactions with the surrounding environment. In the case of bronze, it's primarily the result of the copper reacting with oxygen, moisture, and other substances in the air.
The greenish color of the patina is often due to the formation of copper compounds, such as copper carbonate or copper sulfate, which have a greenish hue. Patina is a natural process and is sometimes intentionally induced to protect metal objects or for decorative purposes. It can give antique or historical items a distinctive and aged appearance.