Your trees could be colonized by lichens.
Lichens often appear leafy or crusty and are colored gray, green, yellow or white.
Typically lichens can be found on dead or declining trees, on rocks, or on the ground. Their presence on failing trees is a sign, but never the cause, of poor plant health. The reduction of plant vigor results in a more open canopy which increases sunlight penetration and subsequent lichen growth. You can remove the lichens from your tree with a stiff brush, but they will probably reappear if you do not determine the cause of decline and correct it. Restoring your dogwood should increase the size and number of leaves and eventually lead to the gradual disappearance of lichens.