Eriophyid mites are the culprits. Here is a good link: Erineum Patches on Tree Leaves

"erineum patches are seldom so severe as to cause significant injury to the overall health of their host trees. Thus, control recommendations are not necessary"

from the link directly above: As with the vast majority of plant galls, erineum patches are seldom so
severe as to cause significant injury to the overall health of their
host trees. Thus, control recommendations are not necessary.

and.. all such problems are solved in fall.. when the leaves fall off ... as long as the leaves stay partially green during summer.. they do continue to do their work of converting sunshine into food ... plagues come and go over the decades ...

some of us hardcore 'anything but green' collectors of variegated plants might consider this a plus ... as it converts an otherwise green tree.. into a variegated tree .. lol ...

ken