The Gaussian Tooth node  (3) defines a blurred spherical (density) area within the cloud layer.

 

In this example I used a Rotation&Twist node (1) and set the rotation Z parameter to 315 degrees.

This way, when you use the Offset node (2) to change its position, X will move the sphere left/right and Y back/forth.

(Of course it depends on the camera angle. It works fine when the angle is set to 90/0/180).

 

The file is here

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