The plant looks like it is not getting enough light. The white stuff is fungus and it is there because the roots are decaying. The soil is not bonsai soil, it looks like dirt. It would need to be repotted. The dead roots will have to be trimmed away and I would soak the plant at least 10 minutes in a fungicide solution. Subdue would be best, but you may have to check what is available for treating soil fungi. Subdue is generally not available to the public. If the roots are badly damaged it might not come back. I would put the tree in a regular pot, not a bonsai pot to allow it to recover if it can. You can use potting soil in that pot, but you will still need to put it in more light and attend to the watering. I suspect you will continue to lose more leaves as the root system will not be able to support the canopy.
Bonsai soil is more porous and contains mostly different grades of fine gravel and some organic matter.
This is a link to bonsai soils.
https://www.bonsailearningcenter.com/IMA ... /Soils.pdf
Also learn to use the chopstick method to determine when to water. More plants are killed by improper watering than anything else.
Most bonsai are trees and most trees need to have a lot of light. It is best to keep bonsai plants outdoors for as long as possible and to provide good light indoors. You may have to supplement with artificial lighting.
Some trees are more tolerant of low light like ficus, and leea (West Indian Holly), others like junipers need to stay outside to thrive.