It looks like you have crepe myrtle aphids.   The second photo looks like shed aphid skins.   If the trees are small, repeated washing with a strong stream of water will keep them under control until the beneficial insects take over.  If you want to try insecticidal soap or horticultural oil, be sure you use them when it is  cool.  A systemic insecticide could be used if the trees are finished flowering to reduce harm to bees.  Safari does not last as long as some systemics and might be a better choice if you feel you have to use a systemic.  Follow directions if you decide to use a chemical.  vw