Robert

It's not a serious health issue...just a cosmetic one.
What you describe occurs frequently on my trees too, and what I do is just take some soapy water and a tooth brush and wash it off. Try to use a biodegradable soap, of course. ...or you can just take a small wire brush and brush it off. This is less likely to occur if your tree is exposed to lots of sunlight...but even with sunlight it can still happen. Don't worry about it, just clean it off.

Robert

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Robert.........in Sta.Cruz