Confidence: I'm as sure as I can be.
Notes: No sign of any foliose lichen, and the blue "crust" of the Lepraria could be better described as "leprose" rather than "crustose". It's very silly to identify a Lepraria from a photograph (often very silly to identify one at all), but the colour, apparent lack of any crust under the granules, and the birch as host all point strongly to L. incana - a very common species on acid bark. As Jenny says in 'Comments' looks like the alga Trentepohlia is there too (T. abietina is often on the same trees as L. incana, though no direct connection). Alan