Dotty, I think what you are describing is algae? Several years ago an older friend told me she uses a light sprinkling of dry powdered Clorox II (or generic store brand, reading labels) on paths to discourage algae and moss (slipping being something she avoids at all costs), no scrubbing, just sprinkle lightly and leave in place. Now I don't know how environmentally responsible that might be long term, but I have used it on paths a couple of times myself and find it doesn't damage nearby perennials - of course you wouldn't let it drift onto foliage when applying. If you have ornamental moss between your stones, I think it could damage the moss though...

Thanks Morz8, that makes sense to me. I worry a lot about slipping and the algae is really slippery, especially on exposed aggregate. They use clorox-type bleach in our drinking water so I don't think it would be environmentally bad unless you use too much. I'm going to try it and if it works, I will post results. Thanks again.

Chlorine bleach is extremely toxic. It's used in minute quantities in drinking water to kill pathogens because the pathogens killed are more dangerous to us than the tiny amount of chlorine needed to kill them. That doesn't mean chlorine must be safe. Spilling diluted bleach water on plants will kill them - been there done that. I would not use chlorine bleach in the garden at all no matter what. You could not possibly use a tiny enough amount to kill the algae without poisoning your soil. Of course the dose makes the poison and you're right that it would not be bad unless you use too much - but too much chlorine is a tiny little bit.

Reg, Clorox II is a color safe bleach alternative for laundry - I don't have a box here just now to check the active ingredient and it's much too stormy to go out!

As for poisoning soil, regular liquid household bleach has an extremely high PH and I could see that causing major problems with heavy or continuous use. But I've added a splash (like 1/2 C or so) to a gallon of water with detergent and scrubbed, rinsed thoroughly, my deck in late winter many years with no adverse effects to lawn grass surrounding it.

Dotty, my cedar deck is in complete shade from Oct to Mar and it can get a bad case of the 'greenies' too - it so rarely has the opportunity to become completely dry in winter.

One of the features of exposed aggregate is that it has sufficient surface texture to avoid being slippery, even with algae/mold. But older surfaces do lose some texture and power washing can accelerate the process, as does bleach.

There are enzyme-based algae/mold removers that are environmentally safe as well as not affecting or damaging surfaces like wood, stone or exposed agg. I've purchased them at the box stores but be sure you get the right stuff - they are typically sold right next to some pretty toxic cleaners for similar purpose, including TSP and various bleaching agents. Coarse builder's sand works pretty well also - I use it on the steps of my deck, which can get treacherous in our wet winters.

Thanks for the discussion -- points are well taken. The cedar deck is much smoother and therefore easier to clean of the green/black algae slime but this cement agg is just nasty. The roughness (and ours is VERY rough) holds onto the stuff and to power wash you have to do it inch by inch. I'm probably looking for a lazy solution that really isn't there, but I think I will try a little laundry bleach on one area, and perhaps a laundry detergent on another to see if anything works. I will also check out the building supply shops to see if they have anything.

Be careful.
Until the bleach or soap type product is rinsed ALL the way off, it will be even MORE SLIPPERY temporarily.

I had a product called "Slimy Green and Slippery Black which was a spray on / spray off cleanser that worked fantastically (way beyond my expectations). I have looked and looked for more and could not find it in stores. When I googled that name today I found "30 Seconds Outdoor Cleaner" available at Rona, Cdn Tire, Home Hdw, Lumberworld, Dick's, Slegg Lumber & Cloverdale Paint. It's a company call Collier Mfg out of Oregon but shipping was prohibitive. Anyway I'm going to give this a try...hopefully is just as effective as we have 600 sq ft of deck in the rainforest on Vanc Isl. Sorry I don't know what the ingredients are to assess the environmental / flora impact.

Here is a link that might be useful: 30seconds outdoor cleaner canada