I had a chick like that.
After 4 days I was too worried that something worse would happen, like the yolk would go septic, or the hole between the yolk and the body would widen as the baby grew...so I decided to fix it.
What had happened in her case was there was a herniation, and though the sac was trying to dry up, the yolk kept it too wet from inside, and she wasn't absorbing it all- a little bit of guts had gotten out into the sac.
I held a warm wet cloth up to it for a long time to soften the out side of it and finally slowly pinched it gently so that the contents went in- this was ****SOOOOOOOOOOOO**** difficult and you must be sooooooooo gentle!!! If there's the slightest tear in that sack, the contents of the abdomen can spill out and you can lose your chick in a horrific way!
When the sac was empty, I held it flat (without tension from the body, so as not to separate it) until it stuck to itself. I then let it back in its solitary confinement to await the sac to drop off. It finally joined its mother and siblings after nearly a week- that was a hard introduction!!
Good luck! I hope you don't have the same kind of herniation!