Supposing that these are some form of lichen, I did some research - it is such a wonderful thing to have the bulk of Man's knowledge at your fingertips!
Guess what? Algae, moss and lichens suffer grave bodily harm when exposed to a particular class of chemicals. Well I guess that part is not a surprise. But this part is: that family of chemicals is relatively harmless to virtually everything else. In fact, it is the active ingredient in Bactine: benzalkonium chlorides.
Now here's the next surprise. No, you don't have to buy 100 bottles of Bactine and distill it to get the benzalkonium chloride - all you have to do is go to the pool/spa section of your local hardware store and buy a bottle of HTH Algae Guard:
|And it was less than $10!|
This is a 30% solution, and yet a 2% solution is supposed to be adequate for killing algae/moss/lichen.
So, the first test is a kind Hippocratic one. Does this stuff harm the Sunbrella canvas on the boat? I uncapped the bottle and put some of the straight 30% solution directly on a scrap of our Sunbrella and left it to dry. After rinsing it out 24 hours later, there was no detectable effect on the canvas. At 30%, the solution has a blue tint - I suspect this is just a dye for appearances, given its intended use. Our canvas is green - if yours is white, you might want to repeat this test.
For the next test, I made up a 6:1 dilution (5%) solution in a hand spray bottle and applied it to a section of the deck with the lichen, and a portion of our canvas which has a liberal infestation of winter algae on the outside. I also applied it to the inside of the canvas directly over our galley vent where we get the most amazing colonies of... well, life I guess, apparently feeding on whatever the vent delivers to the canvas.
The benzalkonium chlorides definitely killed the lichen. But it did take a mild scrubbing with a brush to remove the corpses. Please note that previous scrubbing with the same brush and bleach had no effect on the spots. I apologize that I used a washer for the sizing comparison - I didn't have a dime in my pocket. The washer is a little bigger than a dime and a little smaller than a nickle.
I also applied the 5% benzalkonium chloride solution to a portion of our dodger canvas. Please note that this canvas is more than 10 years old, and has lost most of its water repellant qualities - that's why the mold/mildew can live in it.