It’s important to take care of mildew stains quickly, since mold can rot or weaken material. If your stain is on clothing, wash them with soap and water, rinse, and then dry them in the sun. If there’s still a stain, put a mix of lemon juice and salt on it, and dry it outside. If the mildew is on carpet or a rug, vacuum the area to suck up some of the mold. If there’s still a stain, sponge the area with soap and a clean, damp towel.