Remove the moss. Make sure the pots are up on pebbles or something in those trays because they may well be wicking water up to the roots, effectively drowning them (and you're probably watering too often anyhow, except maybe for the Fukien tea). To provide more humidity, trays must be much wider than pots - regular saucers are only to catch 'watering' water. Yellow leaves does usually mean things are too wet, so back off the frequency and let some of the soil dry between waterings. Junipers are outdoor trees, even in Canada, providing they have proper winter protection, so while it's too late now to put yours outside - it'll be shocked by the weather as it didn't go dormant in Nov. thru now gradually - if it's still alive in Oct. I hope you'll have researched how to take care of it then and have somewhere outside to put it. Do put all out in May if at all possible... and I know what you said, but that's why it's a bad idea to buy them before researching. If those pebbles are glued down, get them off entirely so you can see what's going on with the soil... There's no room here for an instant course in bonsai, never mind for 3 trees, so you'll have to do some learning on your own, though whether or not the trees will wait is another story. Very high light is extremely important for those trees, and you need to look at special fluorescents (not just "gro lites") for many hours a day right over their tops or they won't do well at all. Go look at Bonsai4me.com, look at www.bonsaihunk.com, evergreengardenworks.com for starters, and then go to http://ibonsaiclub.forumotion.com (terrific forum with experts and much more activity than here).

Thanks for the detailed response! I will do my best to keep them happy and healthy and post back to update. Just wondering for the Juniper's new growths at the moment, do I pinch them off or let them grow?

Grow.

Some updates:

After a few months of growing them, it's clear that for me Fukien tea is an easier indoor bonsai for my conditions. I have it next to a bright window, no direct sun. I fertilize it around once a month, it has bloomed for me at least 100 times since I got it back in February (3-5 flowers open up every day), and it is still going strong. It even grew two fruits for me! (are they edible...?)

The brush cherry was doing fine until suffered a heatstroke/insufficient watering and majority of the leaves turned yellow/brown.. I was able to save it but it is not at its best right now for sure..

The Juniper is still doing quite well so far, I know this one will not live too well if inside all year, so I might give it away at some point. Although during the warmer months I open the windows so it can get some fresh air.

I also have a ginseng ficus that started off okay, but lost lots and lots of leaves all of a sudden, they are very sensitive to environment change and I think it took a blow with the fluctuating temperature during spring...

Congrats on everything (that lived :)!