Certified GKH Gardening Expert
Actually, this is a Jade plant. This could be many things, but this can happen due to over watering. This is the most likely cause. As a succulent, this plant requires dry periods between watering. At least the top, but never the entire soil zone.
This article will help with the care of these: https://www.gardeningknowhow.com/ornamental/flowers/rhoeo/growing-rhoeo-plants-in-the-garden.htm