A good natural way of getting rid of a musty smell from a couch is by using some vinegar and water.
Take a spray bottle and fill it halfway with vinegar and then fill the rest of the bottle with some water and then mix the vinegar and water together by shaking the spray bottle.
Then spray the couch with the vinegar and water solution and allow it to sit for a few hours or even a whole day.
Then use an upholstery cleaner with some upholstery cleaning solution to clean the couch with.
You can also sprinkle some baking soda on the couch and allow it to sit overnight and the baking soda will also remove the musty smelling odor from the couch.
You can also use some Febreeze or take the couch cushions outside in the fresh air after using the vinegar, baking soda or Febreeze.