Last week end I watched a show on terrarium building. I saw they put in horticulture charcoal first then garvel then cut a window screen to lay over that , then sand Then the soil and planted plants
and spray misted the container and placed lid on top they also said IF in a couple of days it looks like there is too much moisture droplets in it to off set the lid so air can circulate so mold doesn't develope,, Plus to always make sure the plants
you use have 'clean roots. Not nessecarily rinse off the roots but making sure there is no fungus already present.
I would also add more perlite to the soil for more air circulation