Mold is a double edged sword - it is necessary for the vivarium to run properly, but when left unmanaged, it can be a huge detriment to your dart frog vivarium. Mold, when kept in check, is a beneficial organism that helps to break down waste and other materials. It is natural and all vivariums have some level of mold. However, new vivariums are very susceptible to having mold levels in dangerously high amounts because they lack the necessary microfauna to keep the mold in check. All new tanks should be seeded with springtails and isopods. These tiny helpers are the foundation of your mold control.
Even when springtails and isopods are added, a bloom of mold may be experienced. This is why it is important to allow the vivarium to cycle before adding frogs. Mold can grow fast and in large quantities. When you first observe it, you should add springtails and isopods. If the mold persists in your sphagnum moss, and the springtails and isopods are not able to take care of it, it is time to take action.
Links of interest: Dendroboard.com is a popular forum for many topics regarding keeping dart frogs as pets. The following thread discusses excessive mold in the dart frog vivarium: