Rusty-looking birches

Rusty-looking birches

Observation - Rusty-looking birches - UK and Ireland. Description: Seen on the trunks of silver birches (Betula pendula) but not seen on any other trees, this looks very much as though the trunks have gone patchily rusty! On close inspection with a hand lens the surface looks granular. It tests both C- and K-. In picture

Seen on the trunks of silver birches (Betula pendula) but not seen on any other trees, this looks very much as though the trunks have gone patchily rusty! On close inspection with a hand lens the surface looks granular. It tests both C- and K-. In picture 3 you can see that it is even growing over other lichens. The close-up was taken on a drier day and on a different tree but is otherwise the same.

I had thought this might be one of the Lepraria species and have suggested it as a possible ID, but having failed to find anything like it I'm beginning to wonder if it is algae?