Hey! I don’t know what sort of succulent this …

Hey! I don’t know what sort of succulent this is, but I’ve noticed this brown mark. I thought it was sunburnt, so I moved it into a shadier spot. However, it looks like the mark has grown. Do you know what this is? Thanks in advance!

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This looks like stem rot, friendo. It will keep spreading until it takes over the whole plant. The best course of action would be to cut the stem above the mark, and try to reroot the top half of the plant.