Evernia?

Nitrogen-sensitive
Close-up of Evernia lichen
Evernia is a bushy lichen that is sensitive to air pollution, so is usually found in clean air environments.

It is abundant on branches and twigs of trees and shrubs in the countryside.

Characteristics

Green on top and white on the bottom
Divides evenly into 'forks'
Not to be confused with Ramalina, which is green on both top and bottom and divides unevenly.
Common name
Oakmoss