Algae or lichen?

Algae or lichen?

Observation - Algae or lichen? - UK and Ireland. Description: On the trunk of a well-established silver birch (Betula alba). It is a lichen or, as I suspect, an algae? If it's an algae, is it identifiable? Edit: moved to fungi & lichens in the hope that someone there will recognise it.

On the trunk of a well-established silver birch (Betula alba). It is a lichen or, as I suspect, an algae? If it's an algae, is it identifiable? Edit: moved to fungi & lichens in the hope that someone there will recognise it.