It's established that the mold in the complex is a mutated strain of hay bacillus, and it's implied that the bacteria entity is likely made from it. I believe that this mutationgoes far beyond just decomposition abnormalities, and may be the key to understanding the entities entirely. My theory is that the hay bacillus bacteria has evolved to possess the inherent properties and drives of complex, and specifically it's drive to mimic things from our world.
There is a pattern in the series where we always see the mold growing in areas where a person dies with an entity almost always being within close vicinity. Many times an entity can be found in the very room the death took place. From this I would conclude that the bacteria entities effectively grow from human corpses. My theory proposes that during this process of growth, the bacteria carries with it the idea or concept of the person that they came from, and starts as an extremely crude copy. The bacteria lifeforms look like squiggly drawings that vaguely resemble the human form. They walk and move strangely, and all they do is scream incomprehensibly. However, I believe that as time goes on, the bacteria lifeform refines itself and becomes what we see in Found Footage #3.
We see still life move with much more human movement, and it possesses human mannerism like knocking on doors and turning on lights. I believe that eventually the lifeform may become a near perfect copy of the person that it came from. We also see this refining of mimicry in the complex itself, as it's structures seem to become more complex with time, and we see it begin to create more complicated and abstract things like music. It's not unlikely that the bacteria would reflect this property in the event that this theory is true.
This theory may have far more dire implications than it seems, as I think that it may reveal the major threat that the complex poses. A spooky idea came to my head when reviewing the material related to Peter Tench. The thing that drove him away from his team was the unexplained sound of a crowd of people talking. He would later draw strange imagery of crowds of people around doors after he initially escaped the complex. Is it possible that he saw a glimpse of the future? Is it possible that one day thousands or millions or billions of these fake people will line the halls of the complex? That all of humanity may perish, and all that will be left is a crude memory of what we were? Perhaps that is the great threat the series is leading to.
I think this theory makes a lot of sense from a thematic storytelling standpoint, and seems on brand with the topics Kane Pixels focuses on. The complex seeks to mimic the world we live in and the lifeforms seek to mimic us specifically, which will lead to humanity becoming nothing more than a burning memory. It also has a unique and strong scientific angle that Kane has an affinity for, where the hay bacillus is adapting to it's environment with a warped form of evolution due to the properties of the complex.