I would rather not use it, to be on the safe side. (Regarding: What happens when your pets lick it, what happens when it gets wet in the humidity, is it prone to mold?)
What do you like to build? There are other options that are considered safe, but to recommend something it would be good to know what you like to make. One safe option is e.g. using burrowing clay. I like it for e.g. hides that can be modified by the animal, or to cover the area around a plant. Stone imitations can be made from concrete, or from styrofoam and expansion foam and tile glue (the tile glue is something similar to cement, and the styrofoam and expansion foam must be completely covered by the tile glue).
All of them are considered safe when dried out, and all of them are not prone to mold.
I sometimes combine them, e.g. making a hide from concrete or styrofoam+expansion foam+tile glue to have an "artificial stone", and then add a thick layer of burrowing clay so the reptile can shape it especially at the opening and its base.