You don't mention her age; so, I am assuming that she is not a newborn or very young infant which would strongly influence my advice to you. This finding in a very young infant is concerning for omphalitis which can be quite serious and needs prompt attention.

You seem to be describing some type of infection. More serious accompanying signs of an infection that would prompt the need for medical evaluation include redness (particularly if it enlarges on the abdomen wall), pain, or fever.

You may try a topical over-the-counter antibiotic cream to the area for a few days with the back-up plan of having the problem checked by her pediatrician if it worsens or persists.