Use an inversion of the opaque grayscale layer as a mask on a solid black layer:
Target the opaque layer in Layers panel.
Cmd/Ctrl-click one of the channels in Channels panel.
Press Shift+Cmd/Ctrl+i to invert the selection.
Add a black Solid Color Fill layer - it'll be masked automatically.
(Optionally rasterize and apply mask.)
Hide or delete the opaque layer.
Use the color range command in the select menu to select either just the white pixels or just the black and grey ones. Once your selection is made you can make a layer mask from it (by clicking on the icon of the rectangle with the circle in it in the layers menu) and then refine it using the mask edge feature in the masks panel. Once you have your selection the way you want it you can eith copy and paste to a new layer (if you select the black and grey pixels) or delete (if you select the white pixels).