Lichen is an asset rather than a liability.

Lichen is the oldest living thing on earth, and found almost everywhere.
Flowerless plants, lichen are active in the initial stages of soil
formation, indispensable "starters" of the chain.

They have the ability to change colour like a chameleon and with a bit of
tender loving care and maybe some seaweed fertiliser could add amazing
natural beauty to an otherwise dull spot.

Throughout history distillers, brewers and tanners have used them. They have
produced dyes, perfumes and hair tonic and in recent history have shown to
contain antibiotics.

If you still want to remove these amazing little plants, a water blaster is
recommended.