You could prevent the mould by adding a biocide. Short lived ones (that require topping up) are hydrogen peroxide and bleach (but bleach may be hard on the hands). A long-lived one is copper carbonate.  The Best Way to Eliminate Odor from a Stinky Ceramic Slop or Glaze Bucket - Try to make sure that your waste doesn't get near any aquatic life, especially if you use a long-lived biocide. - I could understand your "wild" clay going mouldy if it has lots of organics in it, but am s