I know some tie it off about half an inch away from the belly button so it dries faster and that way it's less risk from bacteria. But the two I had hatch like this ended up being fine. Then again, I put them in a fresh incubator to dry faster.
That's the top of the shell with what looks like left over membrane. A lot of mine left behind membrane that had consistency of mucus. But all of them were fine. I even had a couple that hatched with the umbilical like that. It dried about two hours after my duckling was dried and floofed...