Having Trouble Identifying My Cactus!
Thanks for your question. While I’m working on plant ID, please let me know the size of the black dot. Thanks,
Thanks for the additional photograph. So, your cactus isn't just one cactus. The top part is actually a different plant from the bottom, and when they are grafted like this, they're called a "moon cactus," and are described in this article. Note that these plants do not live long, typically not more than 5 years, if that.
As to the spot, since I can't look closely enough (which would require a magnifying glass), the best I can do is to refer you to this article that discusses 'brown spots' on cacti. The two I would narrow it down to are the armored scale and anthracnose. You can try both the neem oil (anthracnose) and the imidacloprid (scale) to see if either works. In the meantime, keep this plant away from other plants, lest the problem multiply.
I hope this is helpful. Good luck!