My male cat has had the same issue for a while. I gently clean the inverted nipple, removing the white and/or crusty matter, and rub in a small amount of antibiotic ointment prescribed by my vet. If I miss even one day, the buildup returns. Gabe is 12 and otherwise in good health.
It sounds like a small amount of infectious material leaking out. You should get kitty checked. It could be anything from a blocked "gland" that's infected to some sort of abnormal growth. (Cancerous or otherwise). It's more likely it's a simple form of feline Eosinophilic skin Disease...but get it checked to be sure.