We have used algo clear pro , and softwashed roofs we don’t do a lot of roofs but do do a lot of slate and tile hung walls , generaly softwash them with very good results , not used anything else . I have only experienced the brown marks when using hypo not algo clear pro , it’s a reaction with the hypo to metal ore in the slate if you can put water on it as soon as the hypo mix is applied it will stop this happening but you are then in effect washing off the hypo mix , the same will happen with hypo applied to lead flashing , but if rinsed quickly it won’t mark it