Moisture or sweat is created when there is a difference in temperatures and a humid environment. Warm wet air lands on a colder item and causes sweat. It would not matter what the product was, modeling chocolate or clay. When the substance cannot absorb the moisture it stays on top until it evaporates again.
The way to not let this happen is to keep the modeling chocolate figures the same temperature as the surrounding air, or to keep it tightly covered until the temperatures are the same, then uncover it.
It is just like dew on cold grass that evaporates when the sun comes up and warms the land. Blow cool air from a hair dryer or a gentle fan can also help get rid of moisture that does collect. My website is Chocolate Craft Colors.