I have a white fungus that grew in old mulch around a small tree. It's been a very hot, dry summer and I didn't water the tree/grass much. I dug out the old mulch which came up in sheets down to the dirt and put down new mulch. Its quite hard and extremely thick. It's grown again all around the rim of the garden and now it's spread to my lawn. I took a sample to my local nursery and they couldn't identify it and my web searches have come up empty.
It looks like a slime mold, compare to these pictures: