I agree. If it's discolored, soft, and spreads, it's some kind of rot. I would not begin to know the specific organisms involved. Rotten succulent tissue often smells a little off, which is another pretty obvious clue.
There are 2 main ways that succulents lose their roots, in my experience. They may rot out (the progression defined by the 3 characteristics listed above), often because of overwatering. Or they may die off because of desiccation (lack of moisture in the soil to keep them alive and healthy). So paradoxically a succulent may lose its roots from either too much water or too little water.
Whether the first route proceeds based on a lack of air in the soil or too much moisture in the soil is sort of a moot question to me because those are flip sides of the same coin. Where there is more water, there is less air. And vice versa. Using a fast draining mix more or less ensures there will be more air and less water in the soil, and that's a key (necessary) adaptation for growing Adenium and other succulents which enjoy a lot of water during their active season of growth.
