Lichen is killed by the biocide Benzalkonium Chloride.
I used this on the last panel installation at the recommendation of the manufacturers.
It was the only cleaner that they would guarantee was safe.
The biocide kills the algae that the Lichen lives off.
It was a spray on and leave application.
We were in a hurry so the dwell time was only a few hours.
On a domestic application, I just applied it and left it on.
Over a few months, it killed all the Lichen which was then broken down and dispersed by the elements.