Miles - looks like it could be dried "dog vomit slime mold" (such a tasty name). This will grow on moist carbon materials. It's usually yellow when it's "active" (also leading to the name "scrambled egg slime mold") When it either gets too hot or too dry, this slime mold tends to dry into hard little pancakes that are greyish or tannish in color. I get this mold a fair bit where my drip irrigation systems meets my woodchip mulch. When dried, the pancakes are like little Frisbees.
Subtropical desert (Köppen: BWh)
Elevation: 1090 ft Annual rainfall: 7"