if it's mold it will spread. Isolate and wait is what I would do. If in a week or two there is not more that isn't mold. It's way harder for mold to start to bloom then it is for it to spread.
I would throw it away, and then buy 50 more pounds. And then I would smoke it, before throwing it away.Joking aside, that's the risk of storing tobacco. It's a real lottery.