That egg is definitely rocking! Pipping will be the next stage. You might see a pip by morning or sometime tomorrow. Definitely stop the turning so the chick can get into the hatching position. Hopefully all the eggs are at the same stage and follow suit soon.
Technically you can stop turning eggs at day 15, even if they don't hatch until day 21. But if eggs are starting to rock, you might risk it and stop the turning now. Get all your vents wide open, pipping and hatching requires max oxygen. Get your humidity up by at least 10% or so, somewhere...
Your temps seem to be in a good range. What day did you set them? If this egg was kept in a warmish place before you set them, it could be farther along than it should be too.
It could be a pip, it could also be a scratch on the surface?