That seems to be normal for new vivariums. It should go away after several days if it doesn't or takes over the tank, it might be an issue, but many say that they experience mold with new setups. Mine did it for a couple of weeks and then the mold went away. Some say that springtails will eat the mold which mine seem to do. Either way, you might want to get a colony of springtails started in the vivarium before you put in the frogs, but you are probably just seeing some everyday mold that will go away shortly.