This looks like an infestation of a relatively new pest called Crapemyrtle Bark Scale. They have multiple generations per year, though we are uncertain about particular life cycle details as it's still being studied for our specific area (Maryland and the mid-Atlantic).

Recommendations for control include treating the bark with a dormant rate of horticultural oil and possibly also (or instead) using a systemic insecticide sprayed on the trunk or soaked into the root zone. The systemic would have to be applied by a certified pesticide applicator as such products are not available to the general public and would be unlawful for them to apply.

The dormant oil could be applied by homeowners, but is also best reserved to application by professionals in circumstances where plants are large and spraying needs to occur over one's head. Dormant oil sprays need to be used with careful attention to seasonal timing as well as weather conditions during and immediately after the application, both so they remain effective and so they minimize risks of plant tissue damage. Thorough coverage of all bark surfaces is very important for it to be effective, and more than one treatment may be needed to bring the population under control.

Dead scale insects don't always fall off of their host plant right away, so a treatment may start to work without initially changing the appearance of the infestation. If you spray anything yourself, pay careful attention to the product label directions.

Here is a page on this insect but is geared to nursery professionals.  We are currently working on revising and creating new content for scale insect control which will include this pest, as it is not yet on our existing list of common scale insects. https://extension.umd.edu/resource/crapemyrtle-bark-scale

Also, Scale insects in this group produce sugary excrement called honeydew since they feed on plant sap, and this substance can be attractive to flies and wasps. Beneficial predatory insects (lady beetles and others) can also be drawn to the scale as prey, which fortunately the dormant oil will avoid harming. As the scale die off, the honeydew stuck to the crapemyrtle and surfaces below it will gradually weather off. Sooty mold, a blackish fungus growing on honeydew residue, will also go away in time, if it is present here and contributing to the dirty look of the tree. This fungus does not infect the plant and merely grows on the sticky surfaces, so is only a temporary cosmetic problem.

At this point, you can prune out any cicada damage. If you decide to treat with a dormant oil, you may want to apply two treatments in the dormant season. Apply to all tree surfaces, after leaf drop in the fall, and before bud swell in the spring. A dormant oil is effective at controlling overwintering scales, aphids, and spider mites. It should be applied only if air temperatures are to remain above freezing for at least 24 hours after the spray application.  Follow label directions. 

Applying horticultural oils well can be tricky, which is why professional equipment tends to be more effective and somewhat safer; a professional applicator will also have the protective gear they need to apply it safely, particularly when spraying overhead. Factors impacting how well the treatment works include spray droplet size - fine mists from a high-pressure sprayer coat and stick to the plant surfaces more evenly than larger droplets from most over-the-counter pump or hose-end sprayers. Plus, the container usually needs to be kept well-mixed during spraying so the application doesn't end up too watery or too oily.

  The alternative treatment, a systemic, is a type of insecticide that the plant absorbs and is then ingested by the feeding insects. Since some of these chemicals could also make their way into nectar, they need to be chosen carefully to protect pollinators that visit the flowers. Even though crapemyrtle won't flower again until next summer, some systemic chemicals remain in the plant for months. A professional applicator should keep this in mind and choose a shorter-residual option where possible. Crapemyrtle is not highly valued by pollinators, at least, but honeybees and possibly others will visit the blooms. Cooler weather slows-down insect metabolism and respiration, so the process of the insects succumbing to treatment is a bit slower than in summer, but not to the point where this is a problem. Treating in the "off" season with contact sprays does help to spare numerous beneficial insects, as can systemics since the predators are not consuming the plant directly.

Marian