Plasma residue was a blue-colored byproduct from a starship's warp drive that must be purged regularly.
In early 2152, Zobral clogged the warp injectors of his ship with plasma residue in order to elicit aid from Enterprise. (ENT: "Desert Crossing")
In 2371, the USS Voyager vented plasma residue from its warp nacelles in order to mimic the submissive color-changing behavior of a space-dwelling lifeform. This caused a large creature that had viewed Voyager as a potential rival to cease its attack. (VOY: "Elogium")