Cheese like slime on potato, but no pinkness
It rotted - that's about all you can say from this. Most likely a bacterial rot rather than a fungal rot. If you bought them recently, it was probably already rotting.
This is a form of soft rot, which is a common disease of potatoes world-wide, and can even destroy whole crops in the field. There are a range of bacteria that cause these rots, but Pectobacterium is a common genus for this form of rot.