The green mold on clay is (most likely) penicillin. It would affect you if
you are allergic to molds, but it really is great for increasing the
plasiticity of your clay. The yellow mold is good, too. If you have black
mold, well that is not really mold but the by-product of anearobic
bacteria doing their thing. That is good for plastic clay but it really
stinks (similiar to an outhouse). If you don't mind the assorted comments
from the students, it is great to work with. Make sure they wash with an
antibacterial soap after scraping off in the slip bucket.
Production potters would turn green for that mold growing in your clay.
Consider it a benefit