Moss is a great accent, especially in dart frog vivariums. It will help to process waste in the vivarium, as well as aid in maintaining humidity. Moss is soft and non-abrasive, and will not harm the delicate skin of an amphibian, unlike many reptile substrates. Moss should not be used as a substitute for leaf litter - that is still the best option for ground cover in a tropical vivarium.
Please read the following article on growing moss in a vivarium: