#phlyctis

Phlyctis comes from phlyctidium, an obsolete medical term meaning a large blister. "argentum" is Latin for silver.

Phlyctis argena is a widespread crustose lichen that grows most often on decidous trees but sometimes on stone. More resistant to air pollution than most lichens. To really confirm the ID, though, I'd need to test it with a drop of potassium hydroxide, which will turn yellow and then red.

Most Phlyctidaceae are tropical, and grow on bark.