Removing or eliminating mold
If you have a closed terrarium and it is developing mold the water inside might be too abundant. Remove the cap off the top for a few days to reduce the water and promote some air circulation. This might correct the situation. It is a good option.
Also - if there is limited mold you can use some tools to prune away the affected area. Still keep the cap off to reduce the humidity and water inside.
Also- If one or two plants are affected you might want to just remove them from the terrarium.
While you are removing or pruning plants you should remove any buds of mold that is on the soil. And irrigate the soil a bit by stirring the surface around. Scratch it to get air to it.
