The gray mold is one of the most dangerous of the plethora of slimes, molds, and jellies that you may encounter in a fantasy role-playing game.
The gray mold is nearly invisible in its natural environment. It appears as simply a thin layer of moist clay on the ground (often blending in perfectly with it's surroundings). The only tell tale signs of a gray mold are the many dead bodies that will usually litter the ground around it.
A gray mold feeds on emotion, strong emotions, particularly those of the insane and dying. Now this is not an easy food to acquire, so the gray mold has developed some powers to help cause said emotions. Anyone coming in the area of a gray mold will be affected by it's powers, feeling at first a deep sense of despair. A strong willed and intelligent person can escape, or shrug off the powers at this point (if they realize that the feelings are unnatural). Affected people will lose any will to leave the area of the mold, and will quickly be reduced to a gibbering madman, rolling around on the floor while horrible past events and fictional demons hound them from every side. The mold feeds on this energy, growing stronger (and causing stronger emotions), until the victim finally dies from a combination of starvation, and exhaustion (assuming they don't claw themselves to death first in a fit of madness).
A gray mold is very difficult to kill. The only way to destroy a patch of gray mold is with intense cold, or a magical spell that destroys water. An anti-magic spell of any kind will cause the mold to go dormant for a short period (allowing any victims to be saved if they have not already perished).