Powdery mildew on crepe myrtle
Here is our website on powdery mildew: https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
It can weaken the plant but does not kill it.
If you have a susceptible variety of crape myrtle, this could be a yearly problem, in which case you may want to think about replacement in the future. Allow for proper plant spacing and make sure they are in full sun.
You can also try spraying at the first hint of powdery mildew. It protects uninfected foliage only. Fungicides are preventatives they are not curatives. Check the label registration on horticultural oil products for powdery mildew control listings. They cannot be sprayed in high temperatures.
Consider whether you want to do that when there are plenty of good disease-resistant varieties to be found.