We've used it for the past 10 years as a laundry detergent and it works great, however, I have not used the "Stain Removal" version of this product. I used it extensively on my boat canvas and carpeting both on the boat and in my house using a carpet cleaning machine.
The base ingredients are sodium percarbonate and sodium carbonate. The blend is a color safe cleaner and it will effectively kill mold and mildew. Depending on how bad the staining is, you may need to soak the canvas longer or treat it more than once. Dissolving the powder in hot water is the best approach. I used a 5 gallon bucket filled with warm water then stirred the white powder into a solution until it is completely dissolved. The mixture will look milky white and have bubbles in it.
Spray the canvas to get it wet, then use a pump sprayer to spray the liquid onto the canvas. The longer it remains in contact with the canvas, the better it will work. Doing this on a cool day without direct sun is best and a scrub brush wouldn't hurt either. Rinse thoroughly with water. If you don't rinse it all out, you will see a white residue, simply rinse it again.
I have also used it on my trex deck and it has never killed the grass or the plants.
Good luck.