Mold or rust

I live in a very humid part of Alberta, Canada. I've refinished my 7 year old cedar deck almost every year because it gets little black marks on it that spread over more and more of the board surface over time. They look like small black spots or hairs. They fade when the wood is dry, and turn black when it's wet. I thought it was mold or moss growing on or in the stain (Messmers UV Plus). It contains linseed oil, and apparently some organisms feed off linseed oil. After reading through much of the material on your site, I wonder if it's rust. But how would rust get onto 90% of my deck surface. The hardware is galvanized. I've used aluminum oxide sandpaper. Can that cause it? I think the stain itself contains iron oxide pigments, but that seems to be common in stains. If you could give me an idea of where this might have come from, I'd appreciate it. Also, how can I tell for sure if it's rust and not mold? And if it IS rust, what do I do about it? Thank you!