Perhaps it's user error, but when I alt+click on the procedural grunge, it just displays a dialog box for me to select other procedurals. I can alt+click on the group's mask to display in the viewport but I can't seem to isolate just the procedural?
That's not the mask. That's the map in the channel. The mask is in the layer stack the same as Photoshop. That layer doesn't have a mask. Your mask is applied to the group.
Well, that grunge is not even a mask, it's a fill layer. You can alt-click on the group mask and that will isolate the mask you have there with color selection. If you were to add a Fill layer in the group mask with that specific procedural, then when you alt-click the group mask you would see what you want, the black and white procedural applied in your viewport. Altough, I see your Fill layer is set to Roughness only, what you really want - just hit C until you reach the Roughness channel, and you'll see the Grunge as you want it.
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Perhaps it's user error, but when I alt+click on the procedural grunge, it just displays a dialog box for me to select other procedurals. I can alt+click on the group's mask to display in the viewport but I can't seem to isolate just the procedural?