They are roots. Congrats, your plant is ready to "multiply" for you! Kalanchoe multiply in many ways, making them fun and sometimes wildly prolific. The roots you are seeing are actually often a sign of stress in that spot. If the stem is too long as it has reached for more light, for example, it may sense it will break soon and start growing roots so that when it drops it will have a better chance at continuing its species. It is really not a big deal, but if you find it unsightly increase the light for your plant. That will encourage more compact, robust growth with far fewer roots on the stems.