This is my concern as well and was getting ready to mention it.... Just in case!
that would be awful...fortunately I've never had an issue with mold or aspergillus coming in. The tree companies and the power company come the same day they trim, so what you get has been very fresh...cut the day of. And they dump the whole load, so stuff isn't sitting in their trucks (they don't want to take it back to base).
I would think the mold would come from trimmings having sat in the truck bin or on the ground somewhere for awhile.
Perhaps you can ask how fresh the trimmings are?
LofMc