Why brewed coffee in a pot needs a lid?
Whether solid or liquid, when covering anything you are inhibiting water from evaporating. In this case this is because the air in the coffeepot shares some of the moisture of the coffee, removing the lid allows this moist air to escape and be replaced by dry air, in turn this dry air extracts more moisture from the coffee. Now, knowing that coffee mainly consist of minerals and water, you can conclude that on the surface of the coffee these minerals start to clutter, because less water is present in between them. This explains the visual aspect.
Also you could argue that when the lid is off the top layer of the coffee cools down more rapidly, due to this warm (moist) air escaping. And while the coffee may be hot, it is the outer layer that is cooler and has the typical opaque visuals of a cold coffee.
I am speculating here, but Im pretty sure this is whats happening. I couldnt find anything to reference my ideas but hope its a help anyway.