Mucus
Mucus or phlegm was a bodily secretion expelled by many humanoid species, especially caused by the common cold.

In 2257, the  Linus sneezed in a turbolift full of  and  personnel, jettisoning his mucus into the face of Evan Connolly. ()

Soji Asha could secrete mucus but her "father" Data could not. ()

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Physiology