This is an outbreak of an insect called Crapemyrtle Bark Scale. We have seen them become widespread and abundant after having only been detected in Maryland a few years ago. Information about their life cycle can be found in the linked page, and management options are included in our general page on scale: Introduction to Scale Insects. You can either opt to treat them yourself (as some form of insecticide is usually needed to suppress an outbreak) or hire a certified pesticide applicator (which are usually on staff at landscaping companies or via arborists). Achieving good control of a population might take a year or more to achieve, depending on the methods used and how extensive an infestation was when intervention began. Fortunately, although they can weaken a tree over time and impact its growth and flowering potential, they don't seem to pose an immediate or severe threat to tree health, and sometimes natural insect predators help to keep their numbers in check.

Miri