That effect is actually super easy to achieve believe it or not.
It could be one of two things just eye balling it.
1) Seran Wrap that's just crinkled and set on a roller and just "cycling" in the backround.
2) Woodland Scenics Water on top of wax paper using the same technique as above.
More than likely it's the 1st, considering how consistent it seems to be falling as the amount of detail work that the 2nd option would take, would be quite difficult to achieve in an animation as woodland is typically used for "still picture" dioramas.
I can also tell you with certainty the Stonework on the walls is just cellufoam (Blue foam wall insulation) cut and/or shaped to resemble masonry.