And we have a winner! As Karen guessed, these are lichens on decomposing granite:
This photo is from the Fortynine Palms oasis area in Joshua Tree National Monument. These lichens (symbiotic amalgams of fungi and algae) could be decades or centuries old – they are very slow spreading.
Most creative answer goes to TT, who thought it looked like sweet potato casserole! The quartz does look like marshmallows.