These are what need to be screened out of your soil [You mentioned it looked like 'dirt.] The juniper soil as I recall was double screened- It is the third screening that eliminates the 'fines' from the soil.
The dirt I believe I referred to was what they came in and I threw away. What floated seemes like dust and chaff.
Surprisingly, the juniper soil was not so bad, and much less... er, floated.
I'll tell you what I wound up doing for my ficus. Tell me if I have to redo.
I got rid of bad walmart dirt, rinsed the roots, took some pics, then tried to put them in the new, good soil. When I watered, it all ran off of the new, good soil. When I tried to submerge, half came out of the pot. I wound up getting my very, very fine strainer, just about as small as window screening, and scooping the soil out of the water. I put the soil back in the pot and tried again. Same results. I took all the soil out of the pots and stuck it in the strainer and and ran very fast water over it. I glared at it all for a while. I worried that the stuff running out of the strainer was the organic material I was supposed to keep. I shut the water off and swirled around the soil to see how wet it was. The dusty stuff seemed to still be there, preventing water from soaking into it, so I turned the water back on. I let it get too hot and burned my finger a bit. I went to get an ice cube. When I came back, I shut the water off and threw the soil into the pots. When I watered/submerged, only a little came out this time, so I thought, hurrah. I poked at them with a chopstick for a while. I left them in the water for about ten or fifteen minutes, just to make the soil got soaked. I took them out and put them on the humidity tray.
In the future, I assume it will be easier to do the straining DRY first without trying to mess with the water and the pots and the mess. I did read the thread you referenced. Thank you for pointing it out. I found it very interesting, especially as it made reference to themes I'd seen elsewhere in quests for non-bonsai related information about...well, growing plants in water. As the soils said 2x/3x screened from the site, I guess I thought that meant I was good to go. I'll know better for next time than to believe things are as billed. Grr...
No, It is not normal. You have mentioned that you have only watered these once, What time frame is that in?
For the ficus, about a week. For the juniper, about two.
Scratch your tree in an unobtrusive spot [down to the cambium layer] if you see a pale green color it is fine [for now] any other color indicates you have an ex-tree.
I wasn't sure what the cambium layer was, so I just kept scratching until I found something. It seems mostly white. I guess that's a scratch, then?
Apologies if I sounded gruff at all (or had bad typos/grammar/spelling). Very tired, can't think straight, and heading to sleep now.