Every hot tub is going to get a water line at some point. That looks to me like calcium deposit. Do you have hard water? What happens is that as the water level decreases due to evaporation, the water that was there dries and leaves that residue. Other water lines are from oils on our skin or in our grooming products, but those tend to be brown. Oil and water don't mix so the oils rise to the surface and float there. When they bump into the side walls they stick.
Take away is, I wouldn't worry too much about it. If it is indeed hard water there are products that keep the calcium in suspension to prevent it from depositing on surfaces. They are called sequestering agents. I've had good luck with Defender by Leisure Time.
For oil based water lines there are enzymatic products that will help. There are also oil absorbent "sponges" that work well. I don't think this is your problem though. It really just looks like hard water staining. You can wipe it away pretty easily with a mr. clean magic eraser.
