It looks like gall/midge eggs to me. If you look on the back of most of the Oak trees here, you will find the leaves covered with them. Some gall/midges are harmful, but there are several that are very beneficial and eat spider mites.
I have plants all under my trees and they get covered naturally in the fall and winter with these leaves and I have never had a problem with my plants being infected or chewed up by them.
Yes, they look ugly, but most are not harmful.
I'd be looking around and getting some white index cards and checking your plants for spider mites. Usually the midges won't lay like that unless they are around somewhere.
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