Your soil is all wrong for bonsai or even generally for trees in pots. Doesn't matter if that is the soil it came with; bonsai trees are often sold in potting soil that is very bad for them. Your soil is holding too much moisture and your tree is staying too wet.
You need to repot it in to good bonsai soil, which is much more mineral and free draining. Here's an article about bonsai soil:
and here's one about how to water your bonsai, since you may also be over-watering:
Use the chopstick method to know how wet/dry your soil is and don't water again until it is starting to dry out a bit.
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