That fungus is common. It usually means the soil it staying wet too long and your soil looks spent and it is time to repot. Bonsai are not planted in dirt but a mix that is coarse with different sizes of sand, gravel, and particles with very little peat moss. Drainage is essential. Most bonsai pots only have small drain holes and the roots take up most of the space. Bonsai trees prefer to be outside during the warmer months where they would actually need to be watered almost daily.
Ficus will get used to low light conditions, but looks better in brighter light. You will need to be more careful with watering as the pot still has small drainage holes, but the plants may take a lot longer to dry out.
Where I am cinder is easy to get and I can crush and sift it to get the size I need. For larger bonsai like yours I can even use the cinders as is and put only fine gravel on top for aesthetics. The cheap clay cat litter, not the clumping kind (johhny cat) can be washed of the fine particles and you can use that for part of your mix.
https://www.helpfulgardener.com/forum/vi ... =36&t=3422
