The soil is too wet. Does that pot have a drain hole?
I would repot, your tree looks o.k., so the roots are alive. Make sure you use a mix that drains well and your pot has a large enough drain hole or holes. Do not let any plant sit in water and use the chopstick method to determine when to water. Bonsai media is called akadama and is a mix of mostly different grades of gravel and pumice with only a small amount of organic matter.
https://www.bonsaiempire.com/basics/bon ... onsai-soil
When the weather permits, harden your plant off to spend the summer outside. Bonsai are usually trees and trees really want to be outside as long as possible.
I have found a source locally from the bonsai club, but in the past and for starter plants I still use crushed black cinder which is easy to find here and a lot cheaper. It works and that is really all that matters.
Happy gardening in Hawaii. Gardens are where people grow.