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That's typical of oxalic acid (which someone has branded F9, much like Bob/ PressureTek's original F series of branding... eg F8 HERE  )  I'm not sure if it's a S byproduct that is left behind or what it is for sure...

Since slate is rather porous, you may have to agitate with a brush during a heavy rinse.  The same issue arises with rust removal via ox on asphalt shingles.  (We use these brushes and they'd be perfect for this job as well.)

Not to split hairs, but I highly doubt there was any mold on that roof.  Let's hope not.

Let us know how it turns out and good luck!!

Liberty SoftWash wrote:

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In my professional opinion, you should not use soap on slate. I was told this a long time ago, A surfactant/soap will attract the oil and remove it from the slate, causing a haze to appear. I would bet, it was surfactant not the F9 that caused the issue.

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 This is absolutely correct, never ever use any soaps or surfactants on a slate roof. I have had them pull the oil right out of slate roofs, until I learned my lesson. We simply use SH and water ONLY to clean all slate roofs, and leave the soap home,  for the dishes.

Raystown Roof Cleaning Central PA 1-800-236-0322 wrote:

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Can't help but wonder if you rinsed the acid after applying it? I'm not familiar with the soap drawing the oil out of slate so I can't comment,..but if you didn't rinse really well before the acid dried,..I'd say it's just as likely it's the residual acid that's causing the haze. As stated above, that is typical of oxalic.

Jeff

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 Jeff, I have looked at what that F9 Stuff really is. It is not Oxalic, it is Phosphoric Acid, but like any acid, it can still leave a film, if not rinsed very well, before it dries.