The 'white fuzz' on fresh mushrooms is caused by the release of spores from the mushrooms. This is mushroom mycelium and usually develops when the mushrooms are exposed to certain conditions - this can happen to mushrooms sent via post. The presence of this 'white fuzz' does not make the mushrooms inedible. They are perfectly safe to eat. You may want to remove the 'white fuzz' by hand or by rinsing the mushrooms, but this is not necessary.