Wondering how these peoples' minds work? They either want it done or not. Some college professors have an idealistic way of viewing things,..they want work like this done in some "Magical" way. I had a college professor from Penn State once ask me to kill the growth on the roof,.I was explaining the process, and mentioned I would be keeping everything wet,..and they REALLY didn't want me to get their house wet. I thought they were worried about chemical, and I said, " No, it will be water that gets on the surroundings" And they said they didn't want the house to get wet. I paused, and said what about rain? I didn't wait for a response, and said I can't promise that,..and that was that.
That being said, there really aren't any choices of chemically removing if their concern is plants and fish being poisoned, nothing is going to suit them. Sodium Percarbonate and pressure would be about the only way,...but that's really not a desired method.
I'd give my typical spiel and then move on.
Jeff