Yellow Lichen on Superstition Mountain in the Sonoran desert is likely a combination of Acarospora radicata and possibly A. chrysops.
There are almost 2,000 desert lichens reported from the Sonoran Region, almost 40% of all lichens known from North America.
"The bright neon yellow lichens in the Superstitions Mountains are not one single species. The genus indeed is Acarospora, but there are several closely related species present in the area, often difficult to distinguish. The larger, more exuberant squamulose are mostly Acarospora radicata (previously often called A. socialis, but that is considered a coastal species now). The less exuberant specimens might be A. chrysops, but there are a few others too.
Generally speaking, it is not possible to identify lichens reliably just from photos. There are too many important characteristics missing in a photo. The yellow Acarospora species are among the most notoriously difficult ones. That's ironic since they are such a very conspicuous element of the desert. Some people assume that the large and conspicuous macrolichens should be easier to identify than inconspicuous crusts. Wrong again. In the desert for example Xanthoparmelia are large, foliose macrolichens, but they are chemically very diverse and it's often impossible to distinguish them without knowing their secondary chemistry." Frank Bungartz, Ph.D., Collections Manager of Lichens and Digital Data, Natural History Collections, Arizona State University,USA