Your description sounds like lichens. Lichens grow on plants but are not really parasites of the plants. They live on the dead outer bark and can also be seen growing on fallen branches, rocks, fence posts and old weathered lumber. They often proliferate in a shrub or tree that is declining, as they like the increased light intensity that results from a thinning canopy of leaves. Thus they are often blamed for plant decline when they are more a result of the decline rather than the cause of it. Anything that increases health and vigor (water, fertilizer, etc.) of the trees or shrubs can help reduce the amount of lichen found growing on the plant.
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