Cleaning off plant matter from wood and stone surfaces is generally regarded as a pretty toxic occupation. This is because nearly all the products used are in fact pretty toxic. You can do the whole job manually, but most people are looking for a quicker and easier solution than this, however we would recommend doing any cleaning without chemicals if at all possible.
Any treatments have to be able to kill / remove the green growth and thus must have some level of toxicity to plant life, however there are different ways of achieving this. The product that the Eco Home Centre sells is the Osmo Green Gard. This is a biologically degradable and it also does not contain any formaldehyde, nor chlorine or phosphate. It also has a low odour. So despite being a very effective product it does not have the level of potential for environmental harm that most of the other products on the market have.
So in reality there is no real chemical based way of removing green growth quickly and entirely without risk, however risk can be managed and minimised by making the right choices.