Gray probably does nothing. A natural aging. Green is probably algae more than mold. It's probably best to wash it off with some mold/algae detergent. Ask at a paint store. But I'm still unsure if it's wise to nail it up indoors. You should probably use some kind of primer or similar. I interpret that it has been outside? Then it needs to dry out to indoor climate again, which might make it warped or crooked. If it's just a small amount of paneling, I would rather buy new.
