Ichor was a type of magical elixir, found in a well, but Professor Pimpleton Plumpox experimented with it and it became the corrupt magic sludge.
The Ichor turned Glen McCheese into the Rat Wraith and he now has full possession of it. Lately, he combined it with honey to brainwash bee sprites in a mission.
It appears to be the color purple, since that was the color of the honey and Rat Wrait's well is purple. It is unknown if this relates to the purple pollen in Canal City.