Summary: when stationary, alien blends into background.

Observation: "We hate this. Hate it hate it hate it. It's bad enough that they hide in shadows and run on walls – when they can sit out in plain sight in a well lit room and just wait for you to walk by so they can jump on you from behind … scares the sweat outta ya. You hear stories about someone getting on an elevator alone, closing the door, hitting the button, and just as it starts to move they hear this god-awful chuckle ..."

Thesis: the outermost epidermal layer becomes a crude computer screen, using skin pores as pixels, and secreting pigment for color. When the alien stops moving, it probably "plugs in" to the local bacterial network, and "downloads" the correct background image. As this evolution becomes more efficient, the "pixel" count increases. Kharaa are naturally resistant to most forms of detection – for instance, their body temperature tends to match the room's. With the cloaking evolution, their ability to disappear is total. This upgrade works only when stationary – once the alien moves, or attacks, the link is broken, and their skin reverts.