I would have moved along and said nothing since it was not anything that required parental intervention. If my child heard nothing, I would not bring it to her attention.
If she/he did hear the comments, we could talk about it later, privately, and I'd use that opportunity to have a conversation around the idea that not everyone is always going to like us, and that's okay. I'd also begin to teach the lesson that we don't have to respond to negative comments of others or bite on the "bait" that others throw. Six is not too young to start teaching this.
It's hard not to respond when we see this kind of behavior, especially when we think it is directed at our children, but you did the right thing. Learning to "let it go" is an important life skill, and even though your daughter didn't hear the comments, by choosing to not to say anything to the other girls, you kept the whole episode drama-free.
J. F.