Santa Cruz County, Arizona, US
I think this white growth is a slime mold. I found it on an old log in a mixed conifer forest near a lake, and also found Dog's vomit (Fuligo septica) slime mold in the same area.
Here's Dog's vomit (Fuligo septica), a slime mold growing on a log in the mixed conifer forest. A strange thing is I first found this on the exact same date two years ago in a nearby forest.