Though phlox is notoriously prone to powdery mildew, that’s not the case as far as I can see in your photo here. No, I think the leaves are yellowing through a combination of drought/water stress and shade. You can see the plant is densely foliated – this means that many inner leaves become shaded and hence less effective at photosynthesis. The plant then translocates and stores the energy from the chlorophyll, turning the leaf yellow before it eventualyl drops. From what I can see here, your plant is healthy and there’s no cause for concern.