A quick Google search confirmed my thoughts...
"If it is bright yellow when scraped, all is well, but if it is pinkish, it means that the zinc in the metal has started to leak away by electrolytic corrosion. The seacock should be changed without delay if found to be corroded, as the metal will be brittle and might break off if the handle."
This can be cause by stray current in the marina, actually in the water, where your boat is docked. I'd play it safe and check for this problem and consider installing an isolator.