While the photo shows it on deck, that could be from going upwind and spray o'er the deck. Of course, we don't know what his bow looks like, so it could just only be on the deck. Either way, FSR or the old "any ole oxylitic acid" answer.
looks like rust from the pulpit welds.
Rust from pulpit welds is what I thought also as I am not familiar with any tannins. The hull only got Brown run spots around the Stanchion areas and not all over so I doubt is tannin. What’s interesting on my next leg yesterday from Rhodes to Simi island still sailing upwind with similar conditions did not generate any more of the brown stuff. If it is stanchion rust how to I take care of that?