We have our 22 cruiser out of winter storage and are doing the usual spring clean up/ service stuff. We had the boat covered in a large blue tarp and it seemed to come through winter just fine. However I am noticing blue grey spots on one side of the boat, the starboard side. At first I thought it was mould but after washing the boat, I am not so sure. Could it be some residue from the tarp?. It does come off with a ton of elbow grease . Has anyone experienced this and what did you use to clean it off with?