Oh wow! Didnt see the “crack” didnt look tight until after I read your post and looked at the pictures again. Good call!That is embryo wastes, which is stuck to the chick's navel at hatch. It is usually left inside the egg, but in your case one of the chicks still had it stuck to his/her navel, and somehow hopped over the egg in question and got it stuck onto the egg in the process. I would guarantee you it will just rub right off. It's not from the egg it ended up on at all.
Here are some pics of embryo wastes. It's just what they excrete during the incubation process. (All my pics) It looks like the wastes on your egg just dried out. Mine are all pics taken right after hatch so they're still moist.View attachment 2465237View attachment 2465238