From The Aquarium Wiki
Mulm is usually the unattractive dark brown or black material that settles on the substrate of a tank. It is caused by the waste material ejected by the aquarium animals and left over food and decomposing plants.
In a tank this mulm is slowly digested over months up by the bacteria living there and is broken apart into useful chemicals that plants and other life living in the gravel can absorb.
While non-planted tank owners often remove this material to keep their tanks clean, it is rich in essential chemicals for plants, so planted tank owners rarely remove it.