Hi again, Glenn. I'm just south of you in Sarasota. The other plant next to your philo clump is a Sea Grape
and not much bothers them, they're a native and grow near the beaches as well as lots of other places. Get to be a medium sized sprawling tree.
I would work at getting the shells out from under those plants if you can. Our soil here is naturally slightly alkaline, and so is the water and the shells will gradually make matters worse. Are you irrigating with well water by any chance? Even tap water is slightly alkaline but my well water is more than slightly. We irrigate with untreated well water and in the spring when we have dry spells and have to use the sprinklers more, some of my plants do show signs of iron chlorosis, a condition caused by the higher pH blocking the plant's uptake of nutrients. Even under my oak trees where the soil has been acidified by years of oak leaves, this happens to a degree, depending upon how long the dry spells are and how hot the weather is.
Philos definitely like the soil a bit on the acid side and that may be what is causing the leaves to discolor. The rains we've had this week will help - rain water is close to neutral pH. My advice is "wait and see" if the rain water corrects the situation. Be sure to turn off the sprinklers to those plants whenever you can.
