Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your puppy but congratulations on the newest member of the family.
While I can understand why you'd be worried about the green color, it typically only reflect increased transit time and isn't anything to worry about. This color can sometimes be seen when stools are loose.
As to the reason why they might be on the looser side, there are a number of possible explanations ranging from stress or a change in food although internal parasites are the most common cause.
Even if your pup was previously wormed, puppies are notorious for becoming reinfected and sometimes need to be wormed more than once.
The following is what I'd suggest, especially if the looser stool problem persists:
1. Have a stool sample evaluated if not already done.
2. Consider worming with Panacur and Metronidazole together for 6 days in a row to rule out the most common internal parasites even if a stool is negative. Eggs aren't shed in every stool sample and can be missed.
3. Start a probiotic such as Forti Flora which may be available from local pet or grain stores or can be purchased on the internet.
4. Feed a bland diet of boiled chicken (or lean hamburg) and 20-minute white rice, smaller meals through the day.
I hope this helps and that the stools return to normal in a short period of time. Deb