It could be a sign of a dairy intolerance. My LO started having green mucuousy blood streaked stools at 7 wks old and after an elimination diet (I'm BFing') we found that he's allergic to dairy.soy,egg,and tomatoes. Since I've eliminated his allergens from my diet all of his symptoms cleared up. Like I said, he had green mucousy (really runny too) stools at around 7 wks but leading up to that he had terrible baby acne, ezcema, and silent reflux (mainly terrible nasal congestion). He was a really happy baby, just a little fussy in the evenings but nothing bad. But once I eliminated his allergens from my diet ALL of his problems went away and he's been great ever since. It wasn't over night though, it took a good 4 wks to see everything go away. It took time to get stuff out of my diet, out of his system and his little intestines to heal. You didn't mention if you are FF or BFing, but you could always cut out dairy and see if it makes a difference.
One last thing, half of all babies who have trouble with dairy also have trouble with soy, so you may want to cut that out to. If you're formula feeding you may want to try a formula like Nutramigen or Alimentum...and if those don't work, possibly Neocate or Elecare. If your Bfing, avoid all obvious and hidden dairy to see a difference. It's difficult but you can check out kellymom.com to find a list of ingredients that are dairy or dairy derived. And if you don't see an improvement after 2 wks or so then it's probably safe to assume you LO's problem may be something else.
