based on the pic I got two guesses, copper runoff, it isn’t always green when other crap blends into it, and the second is a tar/pitch that someone put in the gutter to seal it when it started leaking.
From eaco chem:
Use OneRestore undiluted. A copper stain may
show up as blue, green or black depending on
the elements it reacts with to cause the stain.
Generally, an undiluted application repeated
twice before rinsing will remove the stain.
THe main word to focus on here is “Generally” so you may need to do it more than twice. I’d watch the mortar so it isn’t getting hammered too much by pressure. Unless they had their house repointed, that mortar will be old and crumbly.
@Dirtyboy thank you. That’s a good idea on the tar, you could be right. Love eaco Chem products and have reached out to them before and they’re usually pretty helpful. One restore/cleansol seems to knock out pretty much anything sh does not but obviously not every time.
sorry to hear that. I hit a chimney with it before for the copper runoff, it worked ok, but I hate spraying acid above my head. I’m getting a bigger ladder next year (waiting for jan for tax purposes).