I am no expert, but it certainly looks like the yolk sack.
I had a newly hatched chick with something small hanging out of its belly button which I thought was the yolk sack; I was worried about the chick not having absorbed it properly. It was smaller and greyish though, and healed up fine.
The chick was thriving until it was stepped on 3 and a half days later and it essentially popped open. It was a thoroughly traumatic experience, but as it died in my hands, I could see its intestines and all the inside bits that had come out. I believe one of those bits was the yolk sack - which I also believe looks almost the same as the potential-yolk-sack in your photo.
The potential-yolk-sack in your photo has more red areas which also look a bit swollen. But the veiny yellow parts of it look essentially identical.
In regards to the brighter yellow coloured liquid on your chick's feathers, an almost identical coloured fluid leaked from the probable-yolk-sack of my chick.
