The belly button discharge can sometimes be dangerous and sometimes not depending on the type of infection that is causing the belly button discharge.
If the belly button discharge is just caused by a fungal infection then it's not considered dangerous but if the belly button discharge is caused by an umbilical hernia then it could eventually become dangerous if left untreated.
If it's just a fungal infection then using some fungal cream or hydrogen peroxide can help to get rid of the fungal infection.
If it's a belly button hernia and it's left untreated it can become serious and become a medical emergency.
I would recommend getting the belly button discharged checked out by your doctor or urgent care just to be safe.