Well, I have seen something like this before! There was a bathroom "wet wall" on the other side which had two bathrooms back to back. The wet wall was the common wall for the two showers. A leak had developed on BOTH common shower walls so water settled to the slab athen traveled under the 2x4 floor plate, emerging in the adjoining livingroom. The carpet had become saturated and mold grew up the leg of a table which was against the wall.
The reddish color was from the carpet tack strip nails which had rusted PLUS the mold.
Pull up the carpet along the wall. Remove all saturated tack strip and air the carpet. Fix the walls leaks in both showers (remove hard grout and replace with flexible non-molding caulk). Spray the carpet top and bottom with mold killer, let dry. Replace tack strip and reattach carpet.
See if this approach works for you!