It looks like a new body is forming. Normally you will see the new split bodies grow right through an older leaf pair but it doesn't have to happen that way. Most of the "primitive" Lithops (i.e. meyeri, olivacea, naureniae, otzeniana) come from areas with less rainfall and more fog, and so are a little more prone to stacked leaves and sloppier looking growth as a result of overwatering and/ or high humidity.

Eventually older leaves will dry up and become a protective shell. By early summer the old leaves should be gone, unless there is excess moisture - and it doesn't take much for that! It helps to maximize on ventilation, and hence transpiration. Lithops will tolerate windowsill growth, but it takes a little extra effort and know-how to pull it off with long-term success.

By the way, Lithops is the name of the genus. It is not a plural of "lithop".

By the way, Lithops is the name of the genus. It is not a plural of "lithop".

Oops! Thanks, Mark. I knew that didn't look right but I had just read something on line in which "lithop" was used and figured I was wrong with thinking I should have used "Lithops". Turns out, I should have gone with my first thought.

Usually the new leafs grow by the middle, but sometimes they find their way by one side. It this happens the new leaves may or may not take all the water from the old ones.

In my opinion, the green lithops with the new growing will gain width rather than lose height.