You could try adding a few color adjusts under the image, and adjusting each so their subtraction would yield the grey area you need. But I would take the easier way to open the image in PS and draw the area you want populated in another layer, and save that as greyscale mask, with (of course) same location and dimensions. Or if you want more masks for several pops, use an RGB image and put the masks in the channels, and separate in TG again (blue to scalar, green to scalar, red to scalar).