toilet bowl cleaner....
go to Smart and Final (it's a restaurant supply chain) or any store that sells janitorial supplies. Purchase a quart of basic toilet bowl cleaner... it is acid based and should cost less than 5 bucks... purchase a long handled soft scrub brush and a bucket... you now have an algae removal kit... the brown stuff is algae and will come right off with any diluted acid based cleaner... the cheapest is the aforementioned product. Just squirt some in a bucket of water, swab it on, then rinse off with a hose.....
I trailer sailed my NACRA in fresh water for years. The brown stuff was very easy to remove this way. It is a pain without acid based cleaners.