Are you describing a "cave" formed in the side of an elevated area? If so, I don't think moss will grow there unless some light gets in there too. And, I would think water circulation would be very important, to flush out any dying critters, if nothing else. As far as substrate goes, ADA soil is a softer clay substrate, so it can be molded into steeper slopes than Flourite can, and still remain where you put it. People are reporting that it is grows plants a bit better than Flourite, having fertilizers included. And, HC, I'm sure has been grown in Flourite successfully, but planting it is harder because of the bigger grains. Just because a new substrate becomes the favorite of many doesn't mean the old substrate isn't as good as it always was. ADA soil is lighter per cubic inch, so you need to buy less of it for a given volume of substrate, so economic factors may favor it too. Personally, I still like my Soilmaster.