said:

I have green moss growing on my sidewalk, front wall, steps, and the
house wall behind some plants.

A 50% dilution of regular household bleach kills the stuff on the
sidewalk, steps, and front wall. Spray it on, wait half an hour, hose
it off. The part that doesn't disappear (in the cracks) turns a shade
of yellow which I presume means that it's dead and some sweeping and
scratching will remove it.

There are moss killers and preventatives on the market.

One is sold under the Safer brand:
http://www.wormsway.com/detail.asp?sku=MAK705

There's another available he
http://www.bugspray.com/catalog/products/page1623.html
(I've bought other products from bugspray.com and found them
courteous and reliable.)

And these people go into detail about their product (click through
to other pages, including an FAQ):
http://www.anti-growth.com/


I'm loathe to use such a powerful mixture on the house wall because
the run-off will go all over the plants and even if I cover them it'll
end up in the soil possibly killing them (I think). I've tried
(gingerly) a 10% solution but that didn't do anything. The wall is
porous limestone and is painted so it doesn't matter if the mode of
killing causes a stain. I'll repaint it anyway. Does moss require
oxygen to live? If so would painting it with shellac do the trick? Is
there some other appropriate chemical that will kill the moss but not
the plants? BTW the plants are a yew tree and an azalea if that makes
a difference.

The products claim to be safe for use around other plants. The
FAQ for the last cited says:

3. Will Anti-Growth harm plants or shrubs?

A. Anti-Growth will not kill plants or shrubs but can brown the foliage. Plants should
be protected with a ground cloth or sprayed with water to dilute before and after the
Anti-Growth application.


--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)