Fortunately, we don't see any signs of pest insects or disease in the photos. The residue is likely pollen, clumping together from dew or rainfall which wasn't heavy enough to rinse it off of the leaves - especially since it seems to be gathered along leaf veins and near the leaf tips, where moisture would pool as low points on the leaf. (Oaks are finished producing pollen but Hickory and Walnut may still be finishing blooming, and grasses are starting to flower; all of these species have wind-born pollen.) Minor amounts of honeydew from cicadas flying or resting in the trees above may also be allowing the pollen/dust to stick and not rinse off as easily in rain.

See if you can rub it off with a finger or spray it off with a hose. If not, see if you can send in a couple more photos as unfortunately the second close-up is either out-of-focus or too low-resolution for us to see good detail. Is the residue on the underside of the leaves as well? If not, that would suggest this is a temporary environmental condition that should weather-off in time.

Miri