Richard, one possibility is R. polycarpum, which can be 3-septate, has spores hyaline, and medulla I+blue. There aren't many Rhizocarpon species with medulla I+ blue. There is usually a K+ red pigment in the epithecium of this species, but not always, and the exciple should be K-.
The dimorphism in the areoles is curious -- I have no ideas about that. I'm assuming that the chemistry is similar between areole types, otherwise it looks like parasitism.
Bruce