It's hard to tell from the symptoms pictured what the cause is, and more than one factor might be overlapping. The foliage discoloration might be stippling, a type of insect feeding damage that results in pale speckling. It also could be viral in nature, as it's hard to tell if the pale areas are more mottled/marbled or speckled. If stippling is present, a common culprit is the aptly-named Phlox Plant Bug. They can cause heavy cosmetic damage, but generally don't affect long-term plant health. You could check the leaf undersides for indications of any insect activity, though since the youngest leaves appear to be healthier, it's possible the insects have already died out or moved on.

The leaf distortion (curving, twisting, puckering, etc.) might be due to that same insect feeding, or to an earlier aphid outbreak that is now gone, or due to exposure to herbicide drift or even a viral infection. Viral plant diseases are not curable or treatable, and symptoms tend to worsen over time. Insect feeding damage can be pruned out and will eventually go away as new growth replaces or covers-up old damage (or, worst-case, the plant starts fresh in the spring). Herbicide impacts vary in their severity, depending on the dosage of exposure and chemical involved. Sometimes plants grow out of the damage, and sometimes they can't, so time will tell if that is the case here. As far as you know, was any type of weed killer applied to that part of the yard in recent weeks? (Might a neighbor have applied something for weed control in that same time frame?)

For now, just monitor the plant for worsening symptoms, and feel free to send more close-up photos of the symptoms (including both upper and under leaf surfaces) for reassessment. Keep an eye on its watering needs, but no intervention is recommended (or possible, really) at this point to mitigate the symptoms.

Miri