Mounding on Plox flowers
It looks like it is powdery mildew affecting your phlox. Apparently it is a common problem with phlox and we have had the perfect weather for it this year. Oregon State plant disease handbook has a page on Phlox Powdery Mildew which lists cultural practices that can reduce disease pressure as well as chemical controls. Look for sprays with a small “o” for organic, or an “h” for homeowner. These chemicals are available for homeowners to purchase. When using a chemical, read the entire label then follow the directions.
Iowa State University Extension also has a page on Powdery Mildew of Phlox with more information regarding prevention of the mildew.
Hope you find these resources useful in your campaign for beautiful phlox.