Your crape myrtle probably has powdery mildew, which this shrub is very susceptible to in hot and humid weather. Spray it with a mixture of 1 tablespoon of baking powder and a few drops of mild dish detergent in 1 gallon of water. Clean up and discard the dropped leaves. Improve air circulation around the plant by pruning back adjacent plants, if possible. Or you could replace the crape myrtle with a mildew-resistant cultivar like 'Tonto' or 'Zuni.'