It's bark. Cambium is a layer of actively dividing tissue sandwiched between the xylem and the phloem in the stems and roots of plants. As this layer grows and divides, the inside cells become the xylem whereas the outside cells become the phloem. Successive divisions produce what is known as secondary phloem. This is where the bark begins. On the outside of this secondary phloem are three rings of tissues collectively referred to as the "periderm." As a layer of cells called the "cork cambium" divides, the outer layer becomes cork. These cells die as soon as they are fully developed. I's a normal part of your plant's morphology.

Al

Thanks a lot for the informative response! That was really interesting to read about as well; i'll probably look up more info on it too now that I have some terms to search with. I usually have a hard time finding info on my pothos beyond the bare basics so this was really helpful. Thanks again <3

Thought I'd let you know you're not likely to find that sort of information as a set aside specific to pothos. Rather, it will be found in texts or other media that discuss aspects of plant morphology/physiology that's not commonly found in 'houseplant' books or on sites that proffer info related to care of one or more specific species. If you're interested in understanding what makes plants tick, you might try Brian Capon's 2010 3rd edition of Botany for Gardeners. It's a good book at a fair price, and provides a good basic overview.

Al