I buy drywall mud in the 40 lb buckets, but because I'm not working very quickly, one bucket lasts me several months. Now for the problem - after it sits for a couple of weeks, when I take the lid off, there is mold (black, green, and orange) on the surface of the mud. I usually just scrape the mold off the surface and then use the mud. Today, when I took off the lid, there was lots of mold and it smelled like rotten eggs!!! What causes this, and what can I do to prevent it? Is it safe to still use this stuff? Thanks for the help guys!