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No best answer has yet been selected by gardner101. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.It could be what you suspect, salt damage or trafic grime in general, phlox are also susseptable to eel worm, which could be the culprit.
You coud try some plants that do well in coastal areas, that may cope well with salt. Ivy leaved toadflax is pretty and should grow well on your stone wall. Good Luck.