Dropping leaves is a stress response – related either to the environment or to the conditions of the root.
Fungal growth on the soil could be over-watering or poor drainage at the bottom of the pot. Often, having a pot in a pot situation could mean water is trapped at the bottom and excess water is trapping gasses from the soil, like CO2 for example. So the roots are not getting enough O2 through proper drying and watering.
The solution:
First, remove the inner pot and make sure the drainage holes at the bottom are not blocked. If they are blocked or too packed, repot the plant and put a small layer of stones, about 2 cm in diameter at the bottom of the pot to keep the soil from blocking the drainage holes.
Second, elevate the inside pot about 4-5 cm from the bottom of the outer pot, this will allow excess water to leach through properly on the inner pot.