Oh I see, is the stem getting brown and shriveled or is it just the photo? See below. The end cut/top is showing cavity that might trap water and will be a beginning of rot, you might want to flatten that and apply cinnamon. I don't like the look of black dots on the end cut.
You can either cut the top section just below where it is growing (#3) and let it root or you can cut the top in half (#2) then wait for new growth and cut where #3 is. It all depends if you have long enough cutting to make at minimum 6 inches length each cuttings.
Let your cutting callous before planting.
I think if you add well draining potting soil on the bottom, this might grow faster. If you put rocks on top, make sure they don't touch the stem. I've had some of mine got sunburned because rocks got too hot?
