Plasma residue

Plasma residue was a blue-colored byproduct from a starship's warp drive that needed to be purged regularly.

In early 2152, Zobral clogged the warp injectors of his ship with plasma residue in order to elicit aid from Enterprise. 

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.