Your terrariums are very nice!!! Remind me of the first ones I tried. Good job.
On the fungus, it is probably something that came in with one of the plants and not the moss as sphagnum is acidic.
You may want to wash off your plants before planting and
wash off most of the original potting mix as well.
While the general lesson on building a terrarium use a stratified layer of gravel-charcoal-soil mix this approach is really best with those in large aquariums in which some watering may be more regular than would be needed in a smaller setting, such as yours.
I would forgo the gravel and the charcoal and only use enough mix for about a 2 inch covering when pressed down
after planting your plants. Be sure the mix is moist but not wet when doing so. Then only mist enough to allow for condensation to collect on the lid of the terrarium. If no condensation collects mist. If condensation covers the sides and the lid then open up and let dry for a day and close and see what happens.
Also keep in mind that you need a good pair of plant shears as you will have to trim them back if they grow great.
The real challenge seems to be in finding the right plants that are really small growing plants and that offer good eye appeal. There are so many sites on the net that offer so-called mini plants but when you read the list you find that these are regular plants that can get big even in a greenhouse!! Here is a link to one place that offers a variety of tropical plants for terrariums. I recommend checking each plant that is interesting to you out by googling to make sure the plant is what you may want to use.
http://www.glasshouseworks.com/terrariumplants.html

Those that I would recommend are:
Piper, Selaginella, Jewel Orchids

Thanks so much for the advice, I will definitely keep that in mind for the next batch, I lost a lot of height with all the layers. I've ordered from glasshouseworks before and generally been happy, after a longish wait. I didn't know they had a special terrarium listing. I'll definitely check it out. Thanks.