Milton is essentially very dilute toilet bleach (sodium hypochlorite). You have to be careful with it as not only does it bleach, it will cause metal to rust excessively.
I've just used some dilute mosskill (which is used for moss, mould, algae and lichen in greenhouses etc). I've been careful to wash it off the rest of the car as I'm not sure if it has any effect on paint, but I wouldn't have thought so.