It appears like mold, I found a link that goes over some simple ways to test if something is mold.
I personally would check if the wood is soft, if the wood has an earthy smell... like the smell of dirt and decomposing leaves, both strongly indicate mold.
I hope you moved out. The mold you are seeing is likely just the tip of the iceberg. Without performing a full analysis, it is impossible to tell the species of mold involved. That said, there are clearly multiple species proliferating in the structure.
There is a very reasonable chance that there is much more mold inside the walls.
Although you may not ...