Nobody knows about the existence of this mutant. Somewhere in a deep, dark cavern there resided a stationary mutant similar to that of a Nest. It was dying, unable to get enough food to survive. This cavern was entirely unexplored, undocumented, and the only noob, zombie, or even human lifeform that made its way down here was this mutant. Having been left to fester and mutate for centuries, a moldy, bacterial infested fungi adapted to the harsh conditions, evolving to be parasitical in nature and take over hosts to survive.
On some fateful day, this fungi grew across the surface of the mutant, eventually seeping into its brain. Somehow, it managed to survive the toxicity mutant waste populated throughout the mutant, and began its usual takeover process. While not strong enough to fully incapacitate the host and take full control, the fungi managed to create a co-ownership bond with the mutants mind.
Invested with the strength of this fungi, and given the suggestion to expand more, the mutant began a slow, but deadly mutation process into what it is now. The Mold.
The Mold has trouble digging into the surface, unlike mutants such as the Perceptor, and thus it grows its branches along nearby surfaces. It then creates festering fleshy pods which it mutates into dangerous trap-like growths. While it wouldn't attack a living zombie should one stumble upon it, it would consume or absorb its corpse should it get the chance.