The powdery appearance you are noticing is a common fungal disease of crape myrtles called powdery mildew. It is often seen in the spring and fall. Shady, humid locations and cool nights encourage the development of this fungus, as does frequent wetting of the foliage by irrigation or rainfall. It can be controlled with any one of numerous fungicides labeled for powdery mildew and safe for use on crape myrtles.
 
However, the best way to avoid this disease is to use one of the cultivars bred for resistance to powdery mildew, and to plant in a sunny location allowing for free air movement so that wet foliage dries quickly.

Additional Resource
http://edis.ifas.ufl.edu/MG266