Yes, this can happen when a comforter is folded and put away while it is still damp. Prevent mold from growing in your comforter by thoroughly drying it after every wash. The best way to do this is to put it in your machine dryer for a few minutes if the manufacturer says it is drier safe, and then line dry it in a sunny breeze to ensure that it is fully dry. Another way to prevent mold in a comforter is to store it in a cotton pillowcase rather than in plastic. Make sure you store it in a room that is cool and free of moisture also.