The egg takes about 25 hours to go through the hen’s internal egg making factory. Most of that time is in the shell gland adding the shell. After the shell is in place the hen then puts a layer of brown on top if she is a brown or green egg layer.
What almost certainly happened is that she laid the egg early, before the brown was put on top. If you crack one of those other eggs and remove the membrane on the inside so you are looking at the inside of the egg shell, it should be the exact same color. I don’t know why that particular hen that particular time laid an egg early. Often it is because they are startled or somehow feel threatened, or maybe something happened with the lighting if you light the coop, but with living animals I’m sure there could be other reasons. I’ve had some hens do that, though it is pretty rare. You just see a white egg where it doesn’t belong. There is nothing wrong with the egg, it’s safe to eat.
As long as it is not a regular thing there is no problem. She just had a hick-up. There are some diseases that can cause that but the eggs often looked warped, thin-shelled, or just are not right if it is a disease. As long as this is not repeated regularly and the other hens don’t start laying strange eggs, you don’t have that problem. I hate to even mention it because you will probably worry a lot more than you should. That egg looks perfect except the color. I think it was just a hick-up, not a disease.
