If you are worried DEF call the vet.

Also try backing off the switch a bit and see if that fixes it. Sometimes the switch needs to take 2-3 weeks or more depending on the ferret. A few seedy poops is usually not a major concern. Seedy poop usually indicates undigested protein. That can be from food passing to fast because of heath issues, or digestive systems just not adept at digesting so much protein. If the food you are switching to is significantly higher in proteins the belly just might not have adjusted yet.
Calling your vet to ask directly is always best. AND if your ferrets seem otherwise healthy and we’re only talking about a few poops, I wouldn’t get anxious just yet. But call the vet anyway!