Brown gunk - is it mold?

[Lakegeezer]

There is brown gunk showing up in strange places near the water. I'm wondering if anyone else has seen it and knows what it is. I suspect mold, but have never seen anything like it. It has shown up covering parts of a stainless steel boat railing, all over a mooring rope leading out to the boat, some spots on my sail, and on a wooden post of the dock. It is thin, blotchy, and comes off if scraped with a fingernail or knife, but leaves a brown stain behind. Mold makes sense, given the weather we have been having, but perhaps its alien egg pods from mars. Any ideas? thanks...