You could candle that egg again in a few days to see if there is any growth. Chances are, if it's not close to the rest of them, it's a quitter, but I'd candle again to be sure. Or, if you are like me, I can't see much in my Marans eggs, so mark it, and put it in a spot in the bator where you can watch it closely for any ooze or stink. I have yet to have one explode in the bator, but it can happen!
It looked like it was maybe a third of the size of the rest, so I'm pretty sure it quit. I am going to go thru them again in a couple of days and see what's happening. If it's still the same I'll pull it out.