Water can stain a mattress and even though you can't see it through the bedding, you still know the stain is there. Plus, if you ever try to sell your mattress, good luck. Water stains never look good. Fortunately this is a very simple stain to remove. In a matter of minutes, your mattress can be stain-free.
Dampen a soft, clean cloth with water.
Sprinkle a powdered laundry detergent on the stained area. Make sure the laundry detergent doesn't contain bleach--unless the mattress is white.
Scrub the detergent into the stain with the dampened cloth and continue to scrub until the stain is gone.
Vacuum up any residual detergent with the brush attachment of the vacuum cleaner.
Move the hair dryer in a circular or sweeping motion over the dampened area until it is completely dry.