Lepraria sylvicola

Lepraria sylvicola is a species of leprose lichen in the family Stereocaulaceae. This crust-forming lichen grows primarily in woodlands across western Britain and Ireland. It appears as a powdery, bluish-white to pale blue-grey coating on tree bark and occasionally on rock surfaces. The species is particularly common on mature sessile oak trees in natural and semi-natural woodlands, where it often grows alongside mosses and other lichens. The species can be distinguished from similar-looking lichens by its unique combination of chemical compounds, which include roccellic acid and toensbergianic acid. While it occurs from sea level up to 440 metres in elevation, it is most frequently found in sheltered locations on tree trunks and large branches that are protected from direct rainfall....read more on Wikipedia.