Neolara, the exact same thing happened to my DS2. Can't remember all the details but here is a rough idea...
Two weeks old - DS2 started having green nappies.
Three weeks old - Took him to the GP, doctor said he would probably be alright.
Nappies got greener and more mucousy - always liquid and never that "mustard consistency". Also, DS2 fed at two- to three-hourly intervals for much longer that I would have expected.
Eight weeks old - At eight-week check told GP that this was still a problem and got a paediatric referrel.
Eventually DS2 was having six or seven green nappies a day. They didn't smell too bad, but they didn't smell like normal baby poo either.
Sixteen weeks - Started weaning DS2 on baby rice.
Seventeen weeks - Nappies back to normal.
Eighteen weeks - Finally got the paediatrician appointment. A bit late.
DS2 is now coming up to seven months and he is fine.
Here's my advice.
Do not give up breastfeeding. It will only make your life more difficult. It's so tempting to give up, particularly in the early weeks but after a few weeks it will be the easiest thing in the world.
There are two likely causes for the green poo. The least likely is lactose-intolerance. Even if your baby has lactose-intolerance you can still continue to breastfeed him and it will pass in time.
The more likely cause is the one previously mentioned on this thread. Your baby is getting two much foremilk. Because the foremilk is watery it passes through the stomach very quickly and a whole lot reaches the large intestine at once. This produces the same symptoms as lactose-intolerance because your baby's digestive system is being presented with more lactose than it can deal with. Your baby doesn't digest the milk properly producing the green poo, and more worryingly, he isn't getting all the nutritional benefit.
There are a few websites like www.kellymom.com that will explain techniques you can use. You can try feeding him from one breast. Then at the next feed offer him the "used" breast first, then the unused one. You may need to express a bit to prevent yourself getting sore. Don't let the milk build up too much - you don't want a blocked duct. Ouch. There's another technique called breast compression.. When your baby has finished feeding and is just giving an occasional suck-suck-suck (not swallowing much) you encircle your breast with your thumb and forefinger and squeeze hard to squirt a bit of milk into your baby's mouth. Baby will suck and swallow for a while then go back to suck-suck-suck and you do it again. I tried both these techniques with DS2 and it did seem to make things a bit easier.
I am convinced that weaning him at 16 weeks cleared it up. The change in his nappies was so abrupt that I don't think it was just a coincidence. I did check with my HV about weaning him early and her opinion was that it wouldn't do him any harm if it was just baby rice. I think it just gave his stomach something else to work.