The mask isn't for breathing, it's to hide their face so that upper management doesn't have to look at them. They're just that ugly. I looked at the rendered images again and they still strike me as the slig's breathing holes.
If they are descended from swamp ancestors than the positioning would keep the holes above the mud for breathing. It also might explain why they drown so quickly- they can't hold their breath anymore. Plus the pants drag them down.
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