What is causing this

DGJsaver

http://tinypic.com/view.php?pic=s274sg&s=8#.VAQlcfldWXYand how do i clean it off ,simple scrubbing brush and fairly liquid ?

We are assuming water ingress but the pointing between the slabs looks OK..

societys_child

Are we looking at the efflorescence below the lower step?
If so it's usually caused by salts in the bricks.

<clicky>

DGJsaver

societys_child wrote: »

Are we looking at the efflorescence below the lower step?
If so it's usually caused by salts in the bricks.

<clicky>

Sorry . no , i am looking at the green `algae` on the top step mortar

societys_child

Ah, ok, cheap bleach should do it, let it dry naturally - don't wash it off (might need doing a couple of times)