Thanks for the question.

Yes, this does look like powdery mildew. Typically, the damage to your phlox is mostly cosmetic. The plants should survive into 2025. See:

https://www.chicagobotanic.org/plantinfo/preventing_powdery_mildew_phlox - :~:text=The damage is often more,are resistant to powdery mildew.

https://cultivateconnections.org/green-thumb-phlox-what-to-do-about-powdery-mildew/

However if the infection is serious and prolonged, extending over four or five years, your phlox may suffer. For this reason, the following should largely be considered proactive recommendations:

1). Be sure that your phlox plants are properly spaced to allow good air circulation, which helps reduce humidity around the plants. Yours look like they could use some thinning. Do this after blooming is done; probably early September or thereabouts.

2). Water the plants at the base rather than from above to keep the foliage dry. Avoid overhead watering in the evening. A soaker hose for your phlox would be a great investment assuming you don’t already have one. Since your phlox are rather densely packed, water may collect on the leaves, which creates an ideal environment for powdery mildew.

3). Remove and dispose of any affected plant parts to prevent the spread of the mildew. Clean up fallen debris around the plants. This might be tricky as it may mean that large segments of the plant might be removed. Perhaps consider this an early fall (mid-September) project.

4). You could apply a fungicide at this time. Be sure that the product label specifically mentions powdery mildew. A fungicide will NOT remove existing mildew. In other words, don’t expect the white areas of the leaves to disappear after fungicide application. Fungicides prevent the spread of an existing mildew infection. This is precisely what you should strive to do at this time.   Effective fungicides include sulfur, Neem Oil, or potassium bicarbonate. Follow the manufacturer's instructions for application rates and frequency. I suggest avoiding any soap mixture.

Within the genus Phlox are several different species. From your pictures I cannot accurately determine the species you have.  I suspect Phlox paniculata.  In the event, either now or later, you decide to replace things, here are some powdery resistant species: Phlox maculata “Natasha”, Phlox glaberrima “MorrisBerd”, and Phlox paniculata “Robert Poore”, “David”.

Lastly here are some good sites to provide you other tidbits of information:

https://ipm.ucanr.edu/home-and-landscape/powdery-mildew-on-ornamentals/pest-notes/ - gsc.tab=0

https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden

Good luck. Please get back to us with any additional, related questions. Thanks for consulting our forum.