If your talking about the soil having some cottony mold over just the top layer. I've had that happen before, The mold survives on dark and moist conditions(I'm pretty sure), I took that batch of soil, opened it up real wide to expose as much as possible, and left it in front of the window in another room. After about 3 days(and a few mixings), the soil was completely dried out and apparently mold free. I then used that soil(promix BX(no nute value)) without issue.
I am not telling you that this is ok. I took the risk knowing that it may cost me a crop. It didn't cost me my crop that time, But thats no guarantee. I've since started being more careful about soil storage. In fact, my stored soil gets a controlled environment now just like my girls. Low on humidity, heat and a random light on/off timer.