Green poo is not an issue in a healthy, thriving, growing baby.
No idea if it is significant in this case - but a fall from 91st to 9th definitely needs looking at, especially if (as the OP says) the baby is skinny. The green poo may be a symptom of underfeeding here....and if you have been feeding one sided and using a dummy, that's two things easy enough to change :)
It is a BIG MYTH that feeding one sided gets more calories into the baby. The way to increase supply and intake is to switch feed so the baby gets greater volumes, the milk is removed more often, and the breasts make more milk. Feeding one sided reduces milk supply and intake and should only be done deliberately when the mother has an uncomfortable over supply, or if the baby shows this is is clear preference and he is growing just fine.
So - if all that's a concern here is milk intake (and there are no other health issues), getting more milk into the baby needs to start asap
- more frequent feeding, day and night
- consider dropping the dummy so the baby gets her sucking comfort at the breast
- switch feeding - at least both breasts used at each feeding session, and possibly used more than once
- breast compression (google it)
- get latched checked, get baby's mouth checked
Discuss with the HV when to have her weighed next so you can check these strategies are working. If she needs supplementing as well as extra bf, then she can have expressed breastmilk as an option over formula.
But this is not something to ignore at all - it's a highly unusual weight gain, and while some babies gain like this and are fine 'cos that's the way they are, most babies with this pattern could do with more feeding.
Hope this helps.