Is my cactus dying?

[gammastar7]

I got this cactus on a trip to Arizona last week. It's in a tiny pot with a magnet and black rocks, I'm assuming it's carbon/charcoal, are glued around it. I'm new to cactus care so I said, "hey that's cute" and bought two. Had I known what I learned about cacti this past week I would have passed on it. Now one of them looks like this. What should I do?

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