Though it might look tough, tile grout is porous, which makes it one of the most vulnerable materials making up your home. Grout acts like a sponge for moisture, and over time, this permeability can make it the perfect breeding ground for mold. Fortunately, there are a variety of signs that could tip you off about a mold infestation. If you see some discoloration on your grout, you might think it's just dirt from daily use; however, any persistent stains could be the first signs of mold. If the lines feel slimy, woolly, or powdery to the touch, you're likely dealing with an active mold colony.