It has become moldy. You can only wipe it clean with a wrung wet cloth and then blow dry with a hair dryer. In the humid season, put clean and dried shoes into the shoe box, put an anti-mildew sheet on each end, and close the shoe box lid. This way it won't get moldy. Note that if there is a round hole at the end of the shoe box, the hole must be sealed to prevent air flow, otherwise the effect will be poor.