There are two ways to do it.
In the first one make a black canvas layer, put the skull layer on top of the black layer, make a new layer on top of the skull layer and use a cloud or fog brush to apply clouds or fog. These clouds or fog are already a bit transparant so no need to use a blend mode.Do not use pure white for the clouds or fog because they will get lost in the white skull.
I'll show you a second way in a moment...
If you want the cloud and/or fog brushes I can provide them but they are available for free on the net.
Cloud brushes (free):
View attachment Clouds.abr