Answer: I think what you are finding is lichen. It grows by manufacturing its own food through photosynthesis (it is not feeding on your plants) and while it doesn't look very nice, it isn't really hurting your plants. However, if it grows thick enough it can trap excess moisture against the bark of stems and branches and that can cause rot. The best approach is to spray it with bordeaux (lime sulfur) in the winter. This will kill it without harming your plants. It can then be rubbed or peeled off, or you can just let it slough off by itself. Try to increase the air circulation and sunlight to the affected plants. Lichen grows best in cool, damp places.
Best wishes with your shrubs!
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