Baby’s belly button split open?!?!

SmA

His belly button is normal together and what not and I checked this morning with his bum change and it’s ripped open and had a bit of green gunk coming out??! Hes6 months and his umbilical cord was healed by two weeks. I’ll call the doctor tomorrow but should I take him to the er?? Is it okay to wait until tomorrow? And why would this happen? I’m freaking out a bit