You can pick up mould and mildew spray from the supermarket,dettol make one ,you just spray on and leave for a minute before you wipe off . Once the mildew is killed off it will not come back to quickly. They don't have bleach so it won't rot the canvas.
You can use carpet cleaner shampoo as well. Also make sure that you target the mildew affected area with deep cleaning. You can also use bleach after the process. However, you also have to make sure to use a lighter bleach, or else you might lose the color of the canvas. So, look for bleach with an easy compound structure.