Kinda looks like the beginning of a calcium deficiency, they have ready made for sale, search cal-mag. Or you can make it at home, at least the calcium part, eggshells (about a dozen) and vinegar(about a cup). Let it sit for a day, strain the solution of its bigger pieces and boom calcium for your girls.
Another possibility, what does the underside of the leaf look like? It could be mites. Get as close a view as you can, jeweler's loupe can do it, microscope does it better, these little bastards are tiny and difficult to spot. It it easier to see their eggs.
