It looks like some type of fungus. Dont really know what kind. Buti would do a search on white fungus on apple trees and see what you come up with.

Shelf or bracket fungus like that growing on any sort of tree is an indication of internal or heartwood rot. IOW, the tree is dying, albeit rather slowly. It could die completely at any time or gradually over an extended period. I'd think about replacing it in the near future.