Marshmellow-ah is a planet that is inhabited by marhsmellowians or, as Irkens like to call them, Melty goo drops. MarshMellow-ah has no sun, which prevents the marhsmellowians from melting a horrible, terrible, melty, melty death. The reason the Irkens want it is because marshmellow-ah has a large amount of grahm crackers, chocolate and, of course, the marsh mellow people and the marsh mellow trees.
Once the Invader there, Invader Zark, takes over it, the Irkens will add a sun, melt up all of the marshmellowians, add the crackers and chocolate, and enjoy delicous SMORES! Invader D will come along and build a city covering the entire planet and rename it fanonat.