Preserved moss can kill your terrarium. It baffles me why many plant shops sell terrariums with preserved moss inside. It's pretty but not functional.
If you put preserved moss in a closed terrarium with a lid, it will mold within a matter of weeks from the high humidity. This mold can spread to the other plants and result in a dead terrarium. Even worse, the artificially colored mosses can leak their colors when wet, creating a messy soup.
Let's take a peek at these terrariums I saw at a big greenhouse the other day. In the photo below, the terrarium at the top with the lid has some bright green preserved moss inside. You know immediately that this terrarium design will not last- it'll mold within a few weeks.
I've even seen some where the preserved moss is already molding and the terrarium is still for sale!
Unfortunately, preserved moss can't come back to life. The chemicals keep the color nice and vibrant but like a dried flower bouquet, the preserved moss is not meant to get wet.
So would preserved moss work in an open terrarium?
If your terrarium is in an open jar without a lid, the preserved moss will have a better chance of staying dry. You can use it with caution, so long as you water carefully and avoid pouring water over the preserved moss.
Take a look at the photo above again. The terrariums in the bottom of the photo all use preserved moss. Since they get some airflow, the preserved moss isn't an immediate death sentence. However, the only way to water these terrariums is through the opening at the top of the jar. Chances are, when you pour or spray water in, the preserved moss will get damp and mold over time.
If you're set on using preserved moss, I would recommend making an air plant terrarium. Air plants are epiphytic plants that can be placed on top of decorative rocks or sand inside a terrarium. You remove them a few times a month to soak in water, then shake them off and return them to the container. Since you won't have to water the terrarium itself, preserved moss will work!
An air plant terrarium I made with preserved lichen.
What to use instead of preserved moss:
Buy sheet moss that is NOT preserved and simply dried. Check the ingredients on the package to make sure it doesn't contain any chemicals.
Dried sheet moss will come back to life over time when exposed to moisture. Follow the instructions that come with it or soak the moss for a few hours, then wring it out before planting it in your terrarium.
Some people experiment with rehydrating dried sphagnum moss, a common material in the hobby.
Where to get live moss:
Live moss is the obvious choice for closed tropical terrariums. Unfortunately, it can be tricky to get if you're new to the hobby. If you can't find live moss at your local plant shop, check some aquarium and reptile stores. Online groups for plant and terrarium lovers are another good place to go- ask other hobbyists if they have some they could give you for free or at a small price to get started.