Yup aquarium mosses usually work well in a dart frog vivarium, especially on the substrate, because you should be misting about twice daily and keeping the humidity at 80-100%. Other people have done it and it looks really good.
Yeah, I know. Though most like some air flow, so I may see about doing something to have some air flow in the tank. The terrarium will almost be completely sealed, as dart frogs are better off with that since you need to feed usually 3-7 times a week, depending on how much natural food you have. So it would get fresh air from opening for feeding. Not sure if I should use an aquarium or an Exo Terra. I like the benefit of having the front doors.