A good scrub with a soft bristle brush, and soap, not a detergent...Sunlight bar soap would be the best I know of to get the canvas clean. Then, the most important thing is to get it dry, dry dry, and drier. You did not mention if the canvas was treated or not, but even treated, whilst it may not rot on you if put away damp, it will mildew which is even more dificult to clean off than would be the mud. 16 X 16 is not that large, so if you can, find a dry building where it can hang to dry.
Woody