No comments? Please help me I'm worried.

I think it looks to be like a fungus or mold on the soil. My father just potted a christmas cactus into heavy soil outdoor type and it looked very much like yours with some white growth in an area. I removed it and changed out the soil to more of a succulent mix.That would be the best advice i could offer. The best I have found available in my area is BlackGold cactus mix.

Also I am not an expert but the leave growth on the plant looks like a Portulacaria afra. Im sure someone with much more expertise will answer you and guide you to aiding your plant as they have me.

The white stuff on the soil looks like something that grows on potting mix as it decomposes. A more appropriate soil for the plant would be a cactus mix. I have jade and elephant food (portulaca afra) outdoors in zone 9b, and they are not fussy about soil. However, they are rarely watered. A plant in a pot is a whole 'nother story. Use a quick draining cactus mix and don't water too often. I have found Kellogg palm and cactus mix works well for my container succulents. sometimes I amend it with extra perlite.

I agree with Dave that it looks like mold and a Portulacaria.

This is a soil issue and something like gritty mix would be a much more suitable for growing your bonsai. Here is one of my Crassula ovata (different species) and what I am growing it in.

50% granite grit, 50% combination of Turface (clay) and fir bark. This helps provide the roots with lots of oxygen and doesn't retain an excessive amount of water. Which makes it very resistant to mold. =)

First of all, thank you to Dave, lgteacher and ewwmayo :)
I'll change the pot soon so I'll replace the soil with the mixture that ewwmayo described, I guess that will solve the mold problem.
But I still wonder that if the bottom of the shoot in the first photo looks alright to you or not. Because it looks kinda infected or something like that but if it's OK, I guess it's the way it is.
I'll repost when I'm done with repotting and changing the soil.
Thank you again :)

As long as it's not soft/mushy/rotting, it should be okay?

Take your time and getting the right ingredients is important.

Gill has it right most likely - the plants are heavily fertilized by growers before sold, and accumulation of salts from fertilizer is very likely. And gentle brushing as she suggested would get rid of some loose & dry flakes. Then do the 'squeeze test' - gently squeeze the trunk, it should be firm. If soft, there may be some rot (I don't think that's the case).

Do you want to keep it 'small' as bonsai should be? - it is slightly different growing, methods to restrict the growth and keeping plant compact would be necessary. Thick trunk & gnarled branches make it look like an old tree.

If you want a bonsai a trunk chop is in order. .

Precipitates can build up in water-retentive soil that is watered without flushing the soil, such as 'light' watering or frequent misting. Fungus/mold can grow if water-retentive soil is kept wet and dark with little air movement.

I agree with the soil suggestions and watering less often. I would add two things: when you do water, water deeply to allow lots of water(and precipitates) to flush out the bottom. Also, add some air movement if necessary. My indoor plants have a rotating fan on them all day. It distributes the heat from lighting and dissipates surface moisture.

ez

Thank you for the all advises, I've noted them so that I'll be able to apply them :)
About pruning, as I've said I only have this bonsai for almost 6 months and one neighbor said that I should wait so I've been waiting for it. But now that you mentioned it, I've started about reconsidering :)
For the air movement, they are in front of the window so I leave open the window for some fresh air flux, I've never thought that it's not enough.

If there is a local bonsai club to contact, I'm sure they would welcome your attending a gathering and would be glad to provide some hands-on advice for pruning for bonsai. Good luck!

ez