Product went mouldy a week after purchase

I have bought 25kg bag of Montego Classic dog food about a week ago. I have noticed strange discolouration a few days ago, which has got progressively more noticeable. Now the pellets are greyish-green. I no longer have the bag as I normally decant the food into a plastic container. I have been buying this dog food for the past 6 years and I have never experienced this before.