Try spraying it with vinegar, then give it a scrub with a soft brush. Rinse and let dry.
Happy to help!
After you scrub with vinegar and get it clean, check out "wet & forget outdoor". It has some kind of inhibitor that helps to keep this type of issue at bay.
Thank you Kathy for your answer.
After it is cleaned thoroughly, and dried completely, spray the canvas with Scotchguard. This will help it be more water and stain resistant.