Power washers do some amazing things on rocks- if you know someone who has one, or rent one, it might be worth a try.

Maybe lime residue? Vinegar might help, but it would kill any plants (including the moss) as well.

It looks like lichen. I would leave it, looks neat to me. Nice wall BTW.

I agree it's lichen and I think it looks nice and natural. Probably took 50 years to grow.
Catrina

Lots of people would kill for old stone walls covered with moss and lichen.... it helps to give maturity to garden plantings and represents years and years of environmental effects on the wall....instead of fighting it and trying to remove it, try to learn to appreciate it and see how it enhances your plants and flowers! Add some creeping sedums and thymes to fill in between the cracks of the rocks, beautiful!

Just looked at the pic of your steps and wall, how lucky you are to have such a beautiful stone setting for your alpine and rock garden plants!

I agree....it's wonderful.

Yep thats lichen, I agree with the rest. Moss and lichen are natural and beautiful. below is a link if you would like to read it, scroll down to lichen ecology.

Here is a link that might be useful: lichen ecology

Hi. I'm new to this forum and was just scrolling thru and saw this post.

OH MY GOODNESS! I know what you mean about those white spots. I think they are HORRIBLY UGLY and you should be very, very concerned about them. In fact I think you should immediately dig those particular stone steps up and ship them directly to me.

I have a way to...umm,...dispose of them properly and you won't have to worry about them any more, okay?

Just PLEASE be careful digging them up and don't knock off ANY of the gorgeous...I mean horrible, ugly, and probably even deadly lichen. ;)

Suzie