Damped/muted glissando.

Muted glissando is a glissando that is muted at the same time as it is played.

One hand plays a normal glissando while the other hand mutes the strings with the palm of the hand. In order to make a long muted glissando the muting hand must slide along the strings, following the playing finger.

The resonance of the harp's strings is damped thus the glissando sounds muted. 

A damping sign is added above or below the glissando.

The whole harp.