Growbags

I bought some growbags for my tomatoes,chilli's cucumber for the greenhouse last weekend when I opened them they had a white powder on the surface I'm not sure whether to chance using them or not has anyone else come accross this? :confused:

 

Yes, seen it in bags of compost aswell. I just stir it in, guess its some kind of fungus that grows inside the bag. Doesn't seem to cause any problems though.