Need a picture of the whole plant including the pot.

If the leaf fell off in your hand, that means plant central recognized the leaf was not pulling its weight or needed to go for other reasons. The first in a series of 3 events that occurs during leaf abscission (shedding) is resorption, the recovery of nutrients from leaves before they are shed. It's
a key strategy which allows plants to avoid losing nutrients they already paid for. The central element of the green pigment, chlorophyll, is magnesium. The light green mottling is indicative of the fact that magnesium is being reclaimed from chlorophyll molecules for use elsewhere in the plant, so the mottling would be a normal occurrence in leaves in the process of being shed.

That leads to the question - why is it being shed? There's not enough info to do anything but guess, so I'll give you a list and you can check off what isn't the cause. The odds are, it's A) under-watering. If your habit is to water when you can still detect SOME moisture in the soil, it can't be over-watering. B) over-watering, a strong contender for the prize. If you ARE watering when you can still detect moisture in the soil, odds strongly favor over-watering as the cause. C) A major reduction in light between the nursery and the point of sale will cause leaf shedding in ficus. D) It could be a deficiency of one or more nutrients considered mobile in the plant. If any are deficient, the plant will steal nutrients from old foliage to fuel new extension of branches and new leaves, then shed leaves which were robbed. I'd tell you to fertilize, but first you need to tell me if you see any prills (round balls smaller than BBs) of controlled release fertilizer (like Osmocote) on top of or mixed into the soil.

Al

Looks to me like it might be sunscorch.