We discovered this recently on the wall of our rental in the garage conversion "office" in Texas. The place had apparently been closed up and locked before the tenants moved out.

We can see no leaks from the ceiling or on the upper walls.

This discoloration on the wall is not fuzzy, there is no smell, & we treated it with a mildew & mold product, but the stain remains.

We will be replacing the drywall. Is there any way to tell from the photos what this might be, and how can we avoid it in the future?

Thanks from the peanut.