Hi Ross,
Nick is right, you'll have to paint it! Just kidding, seriously I have had the same thing on all my boats. Especially with the lighter color cedar even with six coats of flagship. I was pretty upset the first time I saw it but it didn't take long for the stripe from the strap to fade in with the rest of the wood. I suppose you could cover the boat when it's out in the sun but I expect it will happen anyway because of a lot of woods natural color change after cutting. Think of it like fine quality cherry furniture look forward to the deep rich color change it makes over time.