I've had that happen with new bark. If it's even slightly moist and in a sealed bag the mold grows. I always use it and have never had a problem. It seems once it hits the air, the mold goes away.
If you're really paranoid about using it, dump it in a metal pan and stick it in the oven on a low temp - maybe 200 or so for awhile. I just did that today with some bark I wanted to dry out. It worked great. No smell at the low temps either.