Remove mould from external sandstone wall

[SoMuch2Do]

I have an external retaining wall - concrete blocks which has been clad with thick sandstone tiles and sealed, some 15 years ago.  The wall is now dirty and has what looks like black mould.  I soaked the wall, then scrubbed it with brick and stone degreaser/cleaner, and rinshed it which has helped  but it still shows stain spots. and below after scrubbing 

Any suggestions?

[Jason]

@SoMuch2Do

Hope all is well.

Just wondering whether you've had a chance to try out any of the suggestions above? Hopefully you have a wall that looks like new again!

Jason

[SoMuch2Do]

Sorry @Jason, I have been a bit busy on other things.  I've used the pressure washer and a speciality cleaner solution as suggested.  B.... good workout scrubbing the tiles!  It is difficult to see results until the tiles dry out - several hours or next day, but it is working, albeit slowly. Tiles aren't as black, but they are still stained looking and some have tiny spot moulds. I'll try the pool chlorine solution that was suggested in the next week and report back.

[Jason]

Thanks for the update @SoMuch2Do. Glad to hear that you are having some success.