From what I've learned, and I'm new to this, is that that ring is more than likely a blood ring. (sorry) I had a couple myself in this hatch I'm working on now. I left mine until I was well past the halfway point. Then it was obvious that it wasn't developing as were the others.
Bacteria is the suspected culprit for the bloodrings. The egg has that special coating called 'bloom' (I think) and it protects the eggs as much as possible. Our hands, the oils, acids and bacterias that we carry can affect the eggs too when we handle them. I try to wash my hands well before I handle them.
It's crazy cuz my first egg to pip is a day early, peeping like mad and is the dirtiest! LOL We've nicknamed him Turdy. heehee I did not clean off my eggs before placing them in the bator. (I ordered mine from a member out here) Doing so can cause more problems than just leaving it.
I'd check for a bad odor and/or sweating. I had one "sweat". It's just a bad egg and it's oozing. You get a wet one or a skunky one.... toss it!
I wish you much luck in your hatching.