I parked the boat under a few big trees in mid May to avoid sunbaking my boat cover. We had alot of rain this spring and I guess it got wet under the back of the cover. I have a good cover I just opened up the boat and found two basket ball size black spots on the back inside corners of my cover. I am not sure it just heavy dirt or mold. What would you wash the inside of the cover with to get rid of the black spots ??