Confidence: It's likely to be this, but I can't be certain.
Notes: Probably this, it has the right look (but slightly damp) and it is not fussy about substrate. However, minute characters are important here and a sharp, close photo is needed - not easy even with a professional grade camera and dedicated macro lens under field conditions - hence the rubbish photo on my own site. (Personally, I am waiting to find someone who doesn't mind my hammering a piece out of their wall so I can take some home for a studio shot.) It has tiny patches of thallus (areoles) that are greenish when damp, with bright yellow soredia produced at the edges. Common on walls and bark.