A watering can 50/50 mix of the stronger javel from a brico applied in the evening and left over night will kiil and oxidise off most of any growth then wash off with a bucket of water, nice and clean.
Thanks Corona, but no thanks. My garden is organic and full of wildlife. I have no intention of using bleach where lizards run. I have no problem with the state of our steps other than the crumbling concrete. And please don’t tell me it’s due to the moss and the algae. I have other concrete steps, much much older, covered in greenery and they are in perfect condition. It’s the poor quality concrete that is the problem, so your earlier suggestions are very welcome.
I remember visiting a grand, shabby old house in Ireland and wandering through their garden and being entranced by the long flight of concrete steps, all overgrown with moss and algae.
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Ok understand just be careful in the wet weather can turn very slippery and hips take a long time to repair. No danger to lizards they would stay away because of the smell and then its washed away and unlike other garden chemicals bleach breaks down very quickly. Bleach much misunderstood.
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I’ve always wondered why ancient buildings made of concrete endure for centuries whilst the stuff we have round here crumbles within a a matter of a decade or so - now I know. 
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