It is common.
The most common "grey spots" on the white of the eye are called pingueculae.
These grey spots generally are nasal or temporal and are caused by "actinic degeneration"-- changes from being outdoors in the wind and sun. They can become inflamed and can grow onto the clear cornea and become pterygium
, which could impact vision
and require removal if they get close to the center of the cornea.