I did this during lunch. Started with the blue background. Added the green as a layer. Twisted it using the rotated tool and moved it to its permanent spot.
Added the extra green using the Airbrush on normal (big brush size; medium-high spacing) - mode on Normal.
Added a off-white base for the fog with the airbrush (Bigger brush size - very little - like make-up
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). Mode - Addition; Opacity - 20; Hardness - 38 or lower.
Added the dotty fog with the airbrush: Brush size - different small settings; Hardness - different settings between 55 -75; Mode - Addition; Opacity - different settings - 55+
Denser/ bigger fog masses - bigger brush size and lower opacity again.
Create; Crop; Save; Export; Done in 20 minutes - which I don't think is too bad while still figuring out some stuff.
Thanks @Brieuc_m - had lots of fun!