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?

Kermit

@SoMuch2Do

I would have thought a specialty paver cleaner would do the job. You could also try more natural methods like white vinegar and tea tree oil. I imagine you can't blast the wall with a high pressure cleaner as the slate might chip off?