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What is the image linked to the mask?
What is the image in the layer below it?
As shown above, choose the layer you want to convert to a mask (top layer) and Select > All followed by Edit > Copy.
Add a mask to the middle layer. Then Option click on the mask. The screen will go white. Follow that with Edit > Paste.
Click on the eye in the middle layer and turn off the eye on the top layer.