If you're breastfeeding then it's because she's taking in a lot of the foremilk, which is more sugary and so gives a greener colour. Had this problem a little with my first and hugely with the second. In my experience it's not a problem as they continue to gain weight it just looks strange.
After going to a B/F clinic they suggested expressing the early milk before or just after baby has latched on - no real need to keep milk so just into a muslin or bib - particularly if you have a fast supply. Then pop baby back on and she'll get to the hind milk faster. It's called 'oversupply syndrome'. If you find it keeps happening (green poo) you can try to restrict feeding to just one side per feed. This is perfectly fine and if you tandem feed twins that's what happens anyway, so it's a matter of getting the balance right.