I know exactly what they are, having seen them several times in Hilton Head/Beaufort,SC. I think it might have been on Rose Hill Plantation (you're familiar, probably) that I first saw them...and on a Crape myrtle! Pretty weird, huh?

They are winged and non-winged versions of the Giant Bark Aphid, an insect that I am pretty sure is pretty rare, at least in my experience. It is considered the largest of all the aphids.

Cool. They are almost never a problem because they have a tendency to maintain themselves and not over-populate. I recommend leaving them alone. Maybe even try an experiment: communicate with nature and ask for praying mantises and parasitic wasps. Then keep your eyes open and see what happens. I am a firm believer that these things come in answer to our prayers. Have fun!