Could she be diary intolerant? Might be worth going diary free for a couple of days to see if it helps (HV suggested this when I had the same).
We had this and I was also told it was too much foremilk and try to keep baby on same boob until empty to make sure to get hind milk! It happened when I was still seen by midwives and I sent in a picture and told all normal. Maybe you can check with the health visitor for reassurance?
I've been told it's supposed to be mustard yellow
I'd speak to the health visitor. I was told it's to do with taking more foremilk and not enough hind milk, but it can be other things too. We have had it sporadically so my health visitor wasn't worried as she's gaining weight well. If it's every nappy, I think it's worth checking with someone