This looks like Crapemyrtle Bark Scale, an insect pest we are seeing with greater frequency that was only detected in Maryland very recently -- just a couple of years ago. (It was in southern states for several years before that.) You can find management tips and life cycle information (plus connections to pages that explain what scale are) on the page linked above. Control might take more than one year to accomplish, but fortunately moderate outbreaks probably won't cause the plant serious long-term harm, though blooming might be depressed and growth slowed in the meantime.

Miri