There is a nasty scale insect which attacks Apple trees (amongst others) In Britain it is called American Blight! Though it is not actually a 'blight' which is normally a disease rather than an insect pest. It is the adult form of the woolly apple aphid. The wool is a wax which makes the pest waterproof, so you have to remove the wax to kill the blighter. Spray with a horticultural soft soap and wash the tree down with a good stiff brush, remove as much of the insect as you can, then repeat the treatment until the tree is clear. Do not damage the bark though.
And yes it will spread to other trees nearby.