I'm not great at the diagnosis stuff, but I didn't want to leave you hanging.
Off hand, is it possible that the bubble glass blocks could be forming a lense in spots? It almost looks like those leaves have been burned.
My under educated next guesses are some sort of fungal issue, though ficus tend to resist fungi pretty well; or a nutrient deficiency of some sort.
Like I said, not the best plant doctor in the house, so hopefully someone better will be along soon.