There is no EU commodity standard setting a moisture limit for dried mushrooms. What exists is trade practice, commonly a maximum around 10 to 13 per cent depending on species, and a water activity maximum commonly set at 0.60. The water activity figure is the one that predicts whether moulds can grow.
Why it works this way
Moisture measures how much water is present. Water activity measures how much of it is available to microorganisms, and that is the property that determines microbial stability. A lot can meet a moisture limit and still be unstable if the water is distributed unevenly.
Uneven distribution is the normal condition in this category. A batch containing thick stem pieces and thin cap slices dried in the same tunnel reaches the target average with the slices over-dried and the stem pieces still wet inside. The composite sample passes; the wet fraction spoils the lot from the inside. That is what a conditioning step after drying exists to prevent.
Method matters too. Oven loss on drying at a stated temperature and time, and Karl Fischer, do not give the same number on the same sample, so a limit without a method is not a limit.
What follows in practice
Write three elements into the clause: the moisture method and limit, the water activity limit determined to ISO 21807, and a requirement that both are measured on arrival as well as at packing. Dried mushrooms are hygroscopic, so a breached liner or a sweating container will move the figures in transit.
Then sample from several positions in the pallet rather than from the top carton. Mushroom grading and moisture control sets out the full intake routine.