My girlfriend (a horticulture major) thinks it may be root rot or some other form of pathogen that is travelling from the roots to the leaves due to overwatering or lack of drainage. You’ll notice that the symptoms are most prominant at the base of the leaves and lower leaves suggesting that it is spreading upwards. It doesn’t look like the plant is fighting it very well because normally it will kill and drop some infected leaves. Anyway I don’t know the details, not my field of study but I encourage you to look into symptoms of root rot to see if they match whats happening to your ficus.