Elder whitewash fungus (Hyphodontia sambuci) is a member of a group of fungi known as the resupinates. These fungi tend to leave an external crusty layer on the surface of the infected wood. This particular species displays what you might call a white rot on decaying timber, especially but not exclusively, the soft wood of elder; and always broad-leaved woods, not conifers. If you create a wildlife wood pile in your garden it is pretty likely this fungus, or one of a similar nature, will soon set to work on it to break it down. It is a very common species seen in woods everywhere.
Is is edible? What do you think?