"bmg"- -depending on where you live, humidity may be your problem. I'm in S. Ga. which has high humidity year round. My TT is in the back yard with no cover over it. The unit is hooked up with so the fridge can run and the A/C is set on 78 degrees. With the outside temps exceeding 85 degrees I can tell where every wall stud and roof rafter are just by the mildew growing on the unit. That may just be what you are seeing on top of your unit. To say the metal of your TT is causing the discoloration probably is not true. A temp differential causing mold growth may be the likely problem.