Well I got the bonsai from joebonsai.com so I am not sure exactly what kind of soil they use but it says that it is there top quality soil.
OK the soil is probably not an issue, I was concerned that it was potted in the usual potting soil type mix that is commonly seen in the box stores.
I live in Dayton OH and I have been keeping the tree indoors right by my window
Junipers are not really good candidates for indoor culture but don't rush to put it outside now, wait till spring. After that it should never see the inside of your home again.
I also have a humidity tray under it and have been keeping water in it as well.
Ensure that the water is never high enough to wick into the pot, Junipers don't like to be constantly wet.
The picture that you included does look a little like what could be on my tree at the time it is only on a few branches and maybe only one "spore" on each needle it is not near to the extent of the picture I will try to post a picture asap. These white spots though easily come off by just touching the needle with my finger nail.
If you have what is pictured it is a [url=https://woodypests.cas.psu.edu/FactSheets/InsectFactSheets/html/Juniper_Scale.html]Scale insect.[/url] Remove everyone you can find. You will need to check the tree periodically as this is the adult form, the young ones are smaller and hard to locate until they settle down and become stationary.
[url=https://www.ext.colostate.edu/pubs/insect/05547.html]Insecticidal Soaps[/url] can be used to control the larval stage but will not be effective against the adults, hand-picking is necessary once they form their tough shell.