Save my Cacti, Hi.
 

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 pictu

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