A few days ago there was some yellow-green webbing in my garden bed. It looked like the beginning of some sort of moss, but now I think it must be a fungus. After going away for the weekend I found it had turned into a large, silvery-gray mass that crumbles into dark brown spore-like powder when prodded. It doesn't have a discernable odor, and is matte and dry, not moist. Any idea what this can be?