Saltzgadum was a substance that wasn't normally picked up by the sensors of a Galaxy-class starship.

Furthermore, along with invidium, lucovexitrin, selgninaem, and jakmanite, it was one of only five undetectable substances that can exist in a nitrogen-oxygen atmosphere and that can alter the molecular structure of glass.

Saltzgadum had not been used aboard Federation starships for decades, so it was ruled out as having caused the malfunctions aboard the USS Enterprise-D in 2366. (TNG: "Hollow Pursuits")