"Density" sliders in the left part of the dialog window let you adjust the mask opacity.
In othe words, they make the black areas of the mask translucent.
Since this operation is non-destructive, later on you will be able to restore the mask opacity without any loss of quality.
- So, select a masked layer in the Layers pane.
- Open the "Masks" dialog.
- To reduce opacity of the pixel mask, move the "Density" slider in the "Pixel Mask" panel (screenshot).
- To set opacity of the vector mask, use the "Density" slider in the "Vector Mask" area.
- To adjust density of the smart filter's mask use the respective slider in the "Filter Mask" panel.
- If you need to restore the mask opacity, open the "Masks" dialog once again and set the "Density" value to 100%.