Will drinking water with drywall mud in it hurt my cat?

We are building a new wall in the house, and my boyfriend left a water bucket out overnight. The water has a thin layer of drywall mud in the bottom, and ever since that first night, the cats have been drinking that water exclusively. They haven't touched their water fountain in the kitchen. They aren't acting funny, and since the bucket is really heavy and we'll need it some more, we've just left it out. Drinking that water won't hurt them, will it? Any ideas about why they like it so much? I thought that it might be because it tastes more natural with that rocky stuff in it. Any input?