Here's why:

* Sodium is highly reactive: It reacts readily with oxygen in the air.

* Oxidation: This reaction forms a thin layer of sodium oxide on the surface of the sodium metal.

* Protection: This oxide layer acts as a protective barrier, preventing further reaction with the air.

While the primary component is sodium oxide, there might be other compounds present in the coating due to other reactions with atmospheric components. However, sodium oxide is the dominant component responsible for the pale grey appearance.