unable to select the layer mask of applied tool (motion blur) in smart object

Hi all,

I Have applied a motion blur to an smart object. Wanted to select the layer mask of the motion blur so I can apply a gradient on it. But am not able to. When I click on the layer mask it doesn't select it. Only smart object is selected. My mouse only recognises the smart object.

See the attached picture.

Anyone knows how to fix this?

Thanks