The Wall Thing consists of a set of jaws with teeth/fangs and membranes that appear to be forming a mouth. It is held to the wall by what appears to be a green-bluish sticky substance or tissue which can be stretched out.
The Wall Thing appears to just be decoration: it's clippable, non-interactive, it won't sustain damage, and it doesn't move. It won't damage or attack the player if approached, although some players report that approaching it has resulted in instant death sometimes. However, this might be because of the explosive barrels and oil spill behind the wall that can be ignited if not careful. It is recommended in general to destroy the barrels and ignite the oil spill before proceeding to save Reed.
The nature, purpose and meaning of this creature remain unknown. It is most likely an oversight, a residue left in the game of an unused Thing originally supposed to attack from its position sticking to the wall, like the Tentacle, or produce Scuttlers, like Scuttler Pods.