I don't have much experience with incubating duck eggs, i would have to ask my momma duck Katharina or @WVduckchick or @casportpony for their opinions.
It looks to me like the ring of death…
That looks like a baby that passed in development. After passing, their body becomes a solid unmoving blob and their umbilical blood settles into a ring around them, often called the ring of death. Normal egg development llooksalot different. In a normally developing egg the important things are movement and blood veins that spider from a central source. I would take that egg from your broody hen before it rots/breaks.