Ordinary/regular clay has its share of "bugs" already in the bag when we buy the clay. I've seen tinges of green mold in my bagged BMix clay if left long enough. Usually the clay will dry out before the organisms have a chance to take hold.
Paper clay with its high organic content provides an ideal breeding ground for the mold. Bagged paper clay, if left sitting for an extended period of time, will happily support a flourishing colony and you'll be surprised how fast the mold can spread once it gets started.
Most of the time I see black and/or dark brown mold on my paper clay. It looks unsightly but the paper clay is still very usable. This is only surface mold and it can be easily removed. I take the block of paper clay out of the bag, mix some disinfectant (I like Pine-Sol because of its scent and is not caustic as bleach) with some water and wipe the mold off. I clean the bag throughly with hot water and soap, and spritz some disinfectant into the bag before I replace the now-cleaned paper clay. The presence of the disinfectant will help retard the re-growth of the mold. It will eventually come back so plan to use your paper clay soon.
There is however one kind of paper clay that I've been using for quite some time that DOES NOT show any mold growth. It's the Southern Ice porcelain paper clay from Australia. I do not know what fiber is used, nor if anything else is added as a fungicide, but it wards off any mold growth and has a tremendously long shelf life.
This picture shows 3 types of paper clay. Far left, is my recently reconstituted IMCO sculpture paper clay from my scraps (see earlier post). Top right, shows the Southern Ice porcelain paper clay. I bought that in 2007 and not a speck of mold in it. Bottom right is IMCO sculpture paper clay (from 2008) removed from its bag. The outside is covered with the mold I talked about, but the inside is still good, as shown by the slice.
Both bagged paper clay is a bit on the dry side because water has slowly evaporated from the bag over the years. No worries; add water into the bag and let the paper clay soak up it up.