Answer: What you describe is probably lichen, which is harmless and grows where there is high humidity and poor air circulation. Lots of trees and shrubs develop lichen growth and it eventually sloughs off on its own. You can carefully apply lime/sulfur (Bordeaux) to the lichen which will kill it and you can then peel it off or allow it to slough off on its own. Keep lime/sulfur off the leaves of your bottle brush and oak tree - it may damage them. Spray only the lichen. Good luck with your landscape!
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