Several fungi create a white rot on decaying timber. They can be very difficult to tell apart as, although at a detail level they can be very variable between themselves, to the casual, superficial observer like me they all look much the same! I believe this to be the split polecrust fungus (Schizopora paradoxa). Why? Well it is one of the most common 'white rot' species, it occurs on broad-leaved wood where this example was, and it forms a dense thick white crust like this. There are a couple of dozen similar species but generally less common so, statistically, I am more likely to be right than wrong!
Not an edible species as it hard and crusty.Schizopora paradoxa: the split polecrust fungus