Good guess; the "Flavo-" part is right, because of the yellow-green color, which shows up great in your photo. But all those little white dots ("punctae" or pseudocyphellae) will cause it to key out to Flavopunctelia. And since there are a lot of them, and you are in central California where this species is common, I feel pretty good about calling it F. flaventior. Thanks for including the scale bar!
Don Loarie
2013-04-18
2013-04-18
United States of America, California, Alexander Valley, CA