Do you know any substrates that won't get moldy in a very humid environment? (Other than paper towel and reptile carpet) The last time I tried using dirt, it got moldy, but that could be for a number of reasons
It depends on what species you're using it for. However, some choices include cypress mulch (such as Forest Floor), coco fiber (such as Eco Earth), coco husk chips (such as Repti Chips or Exo Terra's Coco Husk), or fir bark (such as Repti Bark, or Forest Bark). You can also mix these together. In a mix you can also add long fibered sphagnum moss.
Obviously these are only safe for species which can be kept on single ingredient loose substrates. All these substrates, when spot cleaned frequently, changed regularly, and uneaten food is removed, are unlikely to mold in a high humidity habitat.
Another option is to use a pre-made mix for high humidity reptiles, such as ABG Mix, NEHerp's Substrates 1 or 2, or the BioDude's Terra Fauna. You can also mix your own ABG mix from recipes you can find online. Each of these will be suited for different reptiles (for example, one of NEHerps's substrates is better for terrestrial and burrowers as it's "softer").
These can be used like the single ingredient substrates -- spot cleaned regularly and replaced frequently. However, they’re a bit more spendy, and most people use them for bioactive.
Anything with organic content has the potential to mold -- all substrates. However, it’s more likely to mold if it is made up of ingredients that decay quickly. "Dirt" from outside, or even topsoil from the home improvement store, has varying components that will decay at different rates. The substrates I listed take a very long time to break down, and are less likely to get moldy in wet conditions.
One option is to go bioactive, if you can take the time to read about it and research, and can find the proper substrate for your pet. It takes some work to set up, but you add clean up crew such as springtails and isopods, which eat the waste and any mold. In a bioactive enclosure, white mold is fairly normal at first, and will go away as the habitat balances, and the clean up crews mature.
I have some posts about mold in bioactives here:
When is mold a bad sign?
How to reduce mold in bioactive enclosures.