I would definitely ask this on the frog forums as well.

I do know you can routinely scrape the mold off (do this outside). It will grow back but at least you are controlling the amount. I also think some mold is harmless, but I am no frog expert.

If you could substitute PVC pipe, the mold issue would be eliminated. I saw a tree frog cage with PVC pipe. Pretty neat idea really.
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Antaresia maculosa
Antaresia childreni

Mark G