i did a test patch of algoclear pro on this & nothing happened at all ?
Myself I would try a test patch using an sh mix or gel on it first to see if it cleans off before doing down the acid wash route I would have thought If it's just black organic growth then it should not really need an acid wash. Either way best you don't let the two mix on the wall as combined can give off a toxic gas.
It's what I call a heavy duty acid cleaner in my book kev that is why it works the way it does.So in effect it's an acid wash so don't beat about the bush and just say what it is