Are you using ADA aqua soil?
You can use clay mud as well.
I use sediment from a delta of a river which works very well also.
Generally no issues with mold at all with most people.
You need some air exchange, just like growing a terrarium.
See growing emergent Crypts on Jan's web site.
Growing aquatic plant emergent is a very old method and there are many methods, however, few have used their own new aquariums to do this, then flood the aquarium and add stem plants.
I have hair grass growing outside in a small tank, cuba etc might not fair as well, but I do have some of that, Gloss grows very fast under such conditions.
Simple seedling trays with soil work well if you want to grow plants out. The plastic covers from those seedling trays can be used to "dry start" just some areas inside a large tank also.
The method is very flexible really.
Regards,
Tom Barr