What is this plant?
Hello there. I thought this was a straightforward question UNTIL I started researching. What you have is either a cactus or a succulent. Now comes the fun. All cacti are succulents, not all succulents are cacti. The big difference is what are called areoles, cacti have them succulents do not. Some succulents do have spines but don’t have areoles, I cannot tell from the photos if spines are present from the photos it does appear that areoles are present. Without photos of this plant flowering it is hard to call this one. The photo that shows a closer view of the top of the plant has what appear to be some small and dried up flowers on it. These apparent flowers do not appear to be cactus type flowers. The good part is that cacti and non-cacti succulents require the same type of care. I found a good website that discusses the care of these type plants, here is a link to it. https://medium.com/@grdninfoonline/how-to-care-for-cactus-succulent-plants-f62fcd4897de This discusses watering, fertilizer, and growing media. At another site I also discovered that these plants could get too much sun which causes them to discolor especially on the ridges leaving the ‘valleys’ in between green. This could be what is going on with your plant since you did not describe what kind of growing conditions it is growing in. I would recommend repotting your pot in fresh growing medium as described in the above link. While you have it out of the pot examine the roots looking for dark brown or black roots which will tell you if you are overwatering your plant a common problem with these type plants. The repotting should wait until the plant’s normal growth cycle resumes which is most likely in the Spring. In the meantime, these plants usually need less water during the fall and winter. I also found the website for the Cactus and Succulent Society of America, http://cactusandsucculentsociety.org/ It lists contact information for the Oregon chapter where you may find more specific information and find someone who can tell you exactly what type of plant you have.