I'd wondered about guttation when you first posted but when I looked it up in relation to crape myrtle one site specifically said they aren't prone to it, although another showed a picture purporting to be dried guttation crystals on a crape myrtle. I think it's more likely to be insects. If you really wanted to go to town on finding out, you could put something white, like a sheet, on the ground and shake or hit a few branches and see if any insect falls out. You may have a hard time seeing sharpshooters, if that's what it is, while they're still in the plant.
