The brown mold has a passive attack that causes cold damage. Repeatedly attacking a brown mold can cause it to multiply due to your body heat.Eating a brown mold corpse is harmless and has a 3% chance of granting poison resistance or cold resistance.
The green mold has a passive attack that causes acid damage. The corpse of a green mold is acidic to eat.Green molds are marked in the source as conveying stoning resistance, but no code actually reads this flag when their corpse is eaten, so they end up conveying nothing.
The red mold is infravisible and has a passive attack that causes fire damage.Eating a red mold corpse is harmless and has a 3% chance of granting poison resistance or fire resistance.
Most molds can be safely left alone unless they are blocking your path. As you are not likely to possess the required resistance for each of their passives early in the game, if attacking them it is best to use any ranged weapon you have - even rocks or junk daggers will suffice. While your pet can also handle the mold, bad luck with weaker pets can result in them taking more damage passively than they can successfully deal.
Since brown and red mold corpses are not harmful and provide a small amount of nutrition, it's usually a good idea to eat them for the small chance you will be granted an intrinsic.
Mold may also generate on rotting corpses, and mold corpses themselves have a chance of reviving, similar to trolls. Older versions and patches had molds taking on the name of any named corpses they grew on.
SpliceHack allows fungi to generate on rotting corpses, similar to SLASH'EM. SpliceHack also adds moldiers: humanoid molds that can use weapons and other items.
Mold, multicellular organism of the division Fungi, typified
by plant bodies composed of a network of cottony filaments.
The colors of molds are due to spores borne on the filaments.
Most molds are saprophytes. Some species (e.g., penicillium)
are used in making cheese and antibiotics.