This sounds like Crapemyrtle Bark Scale, an insect fairly new to Maryland but which has spread widely in the past few years. Information about its life cycle can be found on the page linked above, and options for scale management in general can be found on the other linked portion of our Introduction to Scale Insects page. Depending on population size, control may involve careful manual removal (so as not to scrape off or cut into the bark) and/or one or more types of insecticide, applied either yourself or via a hired professional.

Miri