My owner manual recommends using something like wool lite. Does it look like mold? You might have to check with a dealer on your brand to see what they recommend. Good luck to you.

+1 on woolite. I did my pup canvas twice a year, for twelve years, with woolite. Once about midway through the season and then again in the fall before putting it away for the winter. Occasionally, I would do it at other times if it got particularly dirty on an outing. It always kept it nice and clean and didn't seem to have any ill effects on the canvas. I'm continuing this procedure on our HTT.

Bass Pro Shops have a citrus sail and canvas cleaner that works great. Cleans and takes care of any mold at the same time.

Another vote for Wool Lite. Amazing how well a gentle detergent like it does.