When you create a layer mask, you will notice a link icon in between the mask and the layer. If you click on that link icon, it will disappear, and it will unlink the mask from the layer.
You can then move the layer or the mask independently from one another. Click on either the layer thumbnail or the mask thumbnail to select the object you want to move.
You can link the mask and the layer again by clicking on the space where the link icon used to be.