This looks like young lichen growth, and if so, it is harmless to the plant. (There are hundreds of species occurring in MD, so their appearance can vary considerably, and it's not uncommon to see more than one type growing in the same colony.) It does not need treatment or removal. In fact, as the crepemyrtle matures further and develops flaking bark, it may fall off at that time because all it does is cling to the outermost layer of bark. Here is more lichen information, if you're curious: https://extension.umd.edu/resource/lichens

Miri