On the threads, it's pipe dope - used to make the joint leakproof. I think the flexible pipe is stainless steel. If you're worried, you can make, or buy some leak detection liquid. Basically some soapy water - look for bubbles. You should be fine. Now, not sure about the rust stains on the tank's outer casing, that indicates a water leak and could be more problematic. Can't tell where it starts.