Baby rats with a dot... eww?

So, if you have read my quesitons before you know my Bianca just had a litter of 13. Wasn't sure until like 3 days before that she was even preggers. She's doing great and all the babies made it. They're all healthy and she's a great mom. She even takes the runt from the rest of the litter, to feed it, if she doesn't think it's getting enough. Well I finally took the babies out of the cage to hold and look at them today, and one of them looks like she/he has a puss spot on it's tummy, where the umbilical cord was. No one else has one and mommma doesn't seem worried. There is no information on the web that I could find. Does anyone know what that spot is and what I should do about it?? Or will my Bianca take care of it? She's done everything else right so far. Oh, and I'm pretty sure I found homes for everyone. I was hard, but I'm pretty sure it's covered. Thanks again everyone, your answers helped so much last time.