Yes, it's a nutrient deficiency, which is what chlorosis usually is. Copper, manganese, magnesum and/or iron are the likely culprits. For lilies, it can happen during fast growth spurts, like in the spring after heavy rainfall. Rather than actually having a deficiency in the soil, it is more likely that the pH is causing the imbalance. Correct the pH, and the problem will probably go away, although slowly.
In fact I have several like this right now and took pics of them yesterday. The first pic has them on the lower left.
Usually it mostly corrects itself as the season progresses. I have found that adding micronutrients to the soil doesn't do much, but if you do a foliar spray before the leaves completely mature, they will take in whatever nutrient(s) they lack. This gives the best results for me. Epsom salt by itself (magnesium only) never did anything for me, but if magnesium is the culprit, then it should work.
I am going through a rainy spell right now, and after tomorrow when the stuff won't get washed off, I'll spray with a combination micronutrient spray. This is the one I happen to use, but anything similar should do.
Even if I didn't do anything about it, it doesn't seem to hurt or affect the lily's health. So if I am too late to spray and be effective, I won't worry about it.
