5Next after you have added the layer mask, right click and Edit> paste.
This will place the copy of the clouds into a floating layer above everything else.
Make sure the layer with the layer mask is directly under the floating selection, and then hit the Anchor button, located next to the Delete button.
This will place the "copy" of the clouds into the layer mask.
After that you can "Apply Layer Mask", a command found in the layers dialog that merges the mask onto the layer.
Note:This "fog" layer should probably be above any images you want the effect to show on.