I have seen them carry the egg cases for a few days too. I think they will produce infertile eggs and or wait for them to be fertilized. Usually they darken over the couple days though. It does look somewhat odd. :study:
This is strictly my opinion. I have seen this with some of my females. I believe its the sack that actually holds the "egg sack" inside the females. My theory is they either push it out after giving birth, dropping an infertile sack or near death. I have looked into it before and never found hard evidence of what it is.