Ok, here's the thing with these jobs,..first don't look at it as though it's your RESPONSIBILITY to clean it,...I use to almost feel that way and do as much as I could to appease the customer,..not anymore. I tell the customer in a respectful manner that this doesn't happen overnight,..and it can't be removed overnight. Scott is correct,..spraying lichen and letting it go away on it's own is the only way to do it without causing damage,..physically removing it will remove patches of roof granules. If the customer wants results NOW,..either walk away,..which is what I would do,...or make them sign something that states they were informed and you're not responsible for patches of missing granules. If they don't agree,.then walk away,...these jobs are the customers' fault due to neglect.
*I know there are contractors here and on other boards that physically remove lichen because they couldn't imagine leaving the customer with a roof full of dead lichen,..well,..to me that's ethically worse than making the customer wait for the results,..that will be MUCH better given time. I'm also fairly certain these jobs are sold by some who tell the customer,.."You're gonna have alot of granule loss, but there's nothing we can do about it",...just so they can get the job by promising instant results,...and to be fair,...some may just not know any better.
**Also, I've dealt with this a few times regarding insurance companies,..talk to them and tell them the procedure,..and they will be happy a professional is doing the work in a manner that keeps the roof surface intact as can be for the situation at hand.
***As far as getting paid,..the customer is informed of the procedure,..and your part is DONE after the treatment and that is when payment is expected. An expectation of time for it to go away can be anywhere from 1-3 months,...but it will begin to look better as soon as it dries up from the spraying,..the lichen begins to shrink and will just flake away.
Jeff