When we wrap our metal clay inside plastic wrap, some of us will brush some extra water on it to keep it moist. This moisture can cause mold to grow on the metal clay. It is harmless and won’t hurt the clay. If you don’t like handling it, you can spray some isopropyl alcohol on it and then wrap it back up in new plastic wrap. This should kill the mold.
I have had no problems with the mold affecting my clay's final form.
Until next time have fun claying around.