Save my Cacti, Hi.

Firstly ill introduce myself. My names andy, im 18. and i love weird and wonderful plants. I.E, cacti.
Ive had a problem with minet hat ive had about 2 years... Firstly it got a small hole in it, like a small crater, now i noticed that the whole of one side has grown mould and gone mush and i assume probably died.
Heres a picture: [image:] The pic is bad quality and just taken on my iPhone, if you want a better one just ask.
Any idea what it may be? i read up but the best description i can find is 'Corking', but even that it doesnt look like.
Thanks, Andy

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