Big Mouth, Most mosses in a pot or a terrarium start out looking like cotton fuzz growing in the pot/terrarium. It will cover many plants and kill seedlings too, but I felt you need to know about this. After it gets to a certain size (spread) the green growth starts, and a healthy carpet of moss appears. It will also "take out" a lot of young seedlings and smaller plants, smothering them to death. It isn't too hard to tell the difference between algae, and common moss growth. Just an FYI.