New bonsai owner HELP

Hello, my mother gifted me a Hawaiian umbrella bonsai for Christmas it has done well until this past week it started showing a white mold type looking layer on the soil, I tried scraping it off but turns out it's all throughout the soil. I'm thinking it might be mineral related since it kinda looks like salt the deeper I dug. I also came to find out that it is wired into the pot so I dug out what soil I could and replaced it with miracle grow garden soil. I also cleaned the rocks in rock tray with vinegar. I also want to note that I saw bats coming up from rocks underneath when I cleaned them. Thank you for reading and thank you in advance for any advice. I really want this tree to last.

Ceceliahorn wrote: I also want to note that I saw bats coming up from rocks.

Bats?

Or gnats? If so, I hope they're not the biting kind!

I believe (strongly) that you have saprophytic fungus. It won't hurt the plant but is a sign of overwatering or poor drainage.
homeguides.sfgate.com/dustylooking-mold-...useplants-41109.html
As far as the gnats I would suggest this site.
homeguides.sfgate.com/kill-fungus-gnats-...dish-soap-87288.html

I hope this helps. I don't want to discourage you but I feel the need to inform you that what you have is not an actual bonsai (yet). It might have been sold as a bonsai but it is actually a mallsai . Don't worry I have a link for that as well. Best thing you can do is keep it healthy, learn how to care for it, and educate yourself on the art of bonsai.
bonsai4me.com/Basics/Basicsdevelopingmallsai.htm