I have a bunch of images (they are in-game screenshots) that are "watermarked" with the GUI (there are controls, stats, etc.) and I want to select the pixels consistent accross all of them. I messed with it in Photoshop and managed to somehow separate the GUI by loading the images into stack and setting modes of all layers to "difference". There must be an easy solution to this.
Difference is one way to do it. After you isolate the pixels, flatten the image, select all, copy, then undo the flatten.
Make a new layer group, apply a layer mask to it, then switch to channels, activate the layer mask channel and paste the (copy from above) into the layer mask channel.
Move all your layers into the group and set them back to normal.
You can adjust contrast, invert the mask, and touch up as needed.