Either that or iron chlorosis? Is it on just the one leaf?
I would keep fertilizing, for sure. If it was fert burn, the ends of the leaves would turn brown. (are they?) Make sure the fert you're using has an iron component listed in the minor nutrients on the label. If it doesn't, go shopping at a good nursery for one that has. Chelated iron is a drastic measure, and expensive to buy plus when I needed it, I couldn't get it in a small quantity either. A soluble fert with iron will probably fix it.
Plumies love humidity, and can take the heat too as long as they get water. Mine get some pretty strong direct sun here and have never showed sunburn. Your smoggy sun in SoCal shouldn't be too strong for them.