Hi hi, so this is my first post. I recently (around two months ago) purchased my first bonsai trees. A ficus and a Chinese elm. The ficus seems to be doing just fine but the elm, not so much. I had initially noticed it dropping a leaf here or there after the leaf turned yellow and thought no big deal, they get stressed from moving from what id read. Unfortunately as far as I can tell a Chinese elm is not an indoor bonsai as the seller led me to believe so I had thought that could be my issue and got a grow light. Fast forward a little and there's a white mold like substance growing on the soil so I figured my problem was actually overwatering and I dialed it back quite a bit. the weird thing though is the ficus has had the same watering schedule and has had what looks like the same white mold substance in a couple spots since I got it but it has never spread. Fast forward even more and I realize what I thought was a friendly spider setting up camp in my Chinese elm was actually signs of a bad infestation of spider mites and the way the leaves were dropping was in line with descriptions of spider mite damage. I've just sprayed both my trees down with neem oil and will repeat weekly but I'm wondering if anyone has suggestions for the mold or further advice for spider mites? I do have a bag of bonsai soil I and could repot. Unfortunately I can't have it outside because it gets down to -50 in the winter here. I'm also wondering if spider mites just live in houses or the tree had them from the start since I don't own any other plants.

Pictures?

I scraped off the top layer of soil in hopes of getting rid of the mold or whatever it is and I sprayed the webbing thoroughly with water yesterday so it really doesn't look as bad as it did but hopefully you can see what I'm talking about

Yup, you have spider mites.
The mold you refer to is actually calcium deposits. Due to hard water and/or the pot standing in water the soil evaporates water and leaving scale to form on the surface. In itself not too big of a problem. It can over time lead to problems like inability of the tree to take up nutrients.

Do you have very hard water? Or is your pot sitting in water? Make sure it does not.
How do you water? Do you water every day a little? With the coarse subatret you will have to water often, but also generously. Which is harder to do keeping it inside (By watering more than the pot can hold, you help flush out part of the salts in the pot, reducing the buildup).

The spider mite.. They are tough. You will need to spray it probably 3 times at the right interval to get rid of the infestation. And you might find you get them again soon after. Make sure you also clean the outside of the pot and the areas where the pot sits when you spray to get any of them lurking on the windowsill. They are a sign of dry windless conditions, which match your indoors placement of the tree. DO you have the option to keep it outdoors? (Depending on what winters you get there though!)

Spider mites are easily defeated by predatory mites Phytoseiulus persimilis.

If you are in Europe it should be fairly easy to get a culture before the frost sets in.

Br
Magnus

Appreciate the replies, I've been watering on a probably biweekly basis just based on if the soil seems dry. Hadn't sprayed the pot and whatnot for mites but I will tomorrow. Unfortunately I live in Minnesota and at it's coldest if can reach -70 here so putting outside isn't an option as I've seen now after already having the tree is the best scenario.