Sounds like you put some of your own tap water back into the bucket? The rotten egg smell can come from an iron-loving bacteria present in high-iron water. We have a little of it in our water.
What I do with left over joint compound is clean up all the mud off the sides inside the bucket, level out the mud on the bottom, and then place plastic over the surface of the mud. This works well at preventing dry-out without the mold problem you can get by adding water. Works for a year or two - then it is time to dispose.