Masking out a cloud to animate

I think the answer to masking is :

You SHOULD select the tree, not the cloud. 

Remove the old cloud, replace it with a new cloud.

Because you have a B&W image, you can use channels to make a selection from a layer that has some crushed values to get those tree shapes. (cmd+click channel)

• make a dummy duplicate layer
• create high contrast around your tree
• go to channels, cmd/ctrl click on Red Channel
• go to layers, select original layer, add layer mask
• animate your new clouds underneath

If you want to animated the cloud. I would mask or delete the full Sky. Add a supper large cloudy shy layer.  Then I would use tween to create frames between two frame I create in the frame animation time line where the two layer are visible so the frames composite is building trees and cloudy sky. Where in one frame I position the clouds  to  one size of the composite and fill the Canvas area and in the second frame I move the cloudy sky layer to the other side but still fill the canvas sky area.  Tween then will the animate the move and the clouds will pan in sky in the animation.