There is plenty of natural selection pressure on not being eaten. If a mold or something could evolve to live off of a dry atmosphere plus sunlight and whatever minerals it can eek out of rocks, it could blanket millions of square kilometers of desert. Presumably other living things would find it hard to digest because its protein and chemical structures are so different. Nature is constantly finding weird little niches at the edge of sustainability. Waterless deserts should present an enormous opportunity for something to fill, but it hasn't happened.