The mold isn't killing your Irish moss. It is merely feeding upon what is already dead. The mold is harmless, only an opportunist.
I've never been able to grow true terrestrial moss for any length of time in a dart frog vivarium except for Java moss. The various mosses will start out just fine, but then succumb to either too much humidity, too little light, too little ventillation, or seasonal die backs, etc. Since your goal is to make dart frogs happy, not moss, per se, I'd suggest trying some Java moss. It grows on both wet soil and under water, and has no particular light or ventilation requirements. It will cover the substrate with a nice greenery without competing with your plants. It's a little shaggier than other mosses, but does the job.
You mentioned blooming. Moss does not "bloom," in the usual sense of having flowers. You may have seomthing called Helixine soleirolii, which is also called Irish moss, but is not a moss at all. This is related to the pileas, and may still survive in your terrarium in a more dry portion of it. Some Selaginellas are also called "Irish moss," but they shouldn't be troubled with any of the above limitations. These are usually taller, more like small ferns, and put up with low light and high humidity.
In any event, if you want to have a low mossy ground cover, the Java mass will be your best bet.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
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