These are a common type of mite-produced leaf gall. They may look alarming when abundant, but do no significant damage to the plant. Some years they can be abundant while in other years they can be nearly absent, so an infestation one year won't necessarily repeat itself. You can learn more about this type of mite gall on the linked Ohio State article (the buttonbush photo example is at the bottom). No intervention is necessary.

Miri