The dreaded tarp stain. It happens....but I don't know how. Anytime the tarp stays in long contact with composite material, you'll get it. Happened to one of mine even though I knew about it and tried to prevent it (tarp sagged over winter). Bottom line.....leave it uncovered for a while in the sun (many days) and it should "cook" back out. I tried buffing etc with various things....no go. It just took time. Pretty sure it's a deep moisture issue....within the layers....not a surface thing.