A.E.
Sometimes when a child is constipated, their large intestine secretes a mucuous sort of slime to allow the stool to pass through easier and it can be found in their poop.
Also, depending if she is coughing or not she might have a slight cold or allergies and sometimes there can be an over production of mucus which sometimes can pass through the colon into the childs stool.
If you want to call the pediatrician so you can have some peace of mind then do that. She does not need to be rushed to the hospital though, if you say she is eating normal , in good spirits, not lethargic, no vomiting etc then there is no cause for concern. I hope this helps.