Brown but hardened stem?

Hello, I’m kind of new to propagating plants and plant care, so I may not fully understand all the lingo or what to research, but I’m worried if this plant is rotting? Or if this is just its natural process of sorts.

For the plant, I currently have it in a bottle with tap water which is changed out every week (or whenever I notice something may be off?), and before refilling the bottle, i clean it and the bottom plant area with some hydrogen peroxide.