I have lichen all over my sidewalk. It seems to only be on my part of the sidewalk on my street. I do have some trees that overhang and cause lots of shade. What is the best way to remove this stuff and prevent it form coming back? I was thinking I could power wash it away.
Power or pressure washing is the only way to remove this growth. What I think is causing this is excess fertilizer. From washing out of your grass and over the sidewalk or after fertilizing, you aren't blowing the fertilizer off your walk. Do any of your neighbors have this on the walks beside their lawns? I've never encountered this before but by looking at your pictures, the most growth is closest to the grass where there would have been more fertilizer left after spreading. Be careful with the water staying too long in one spot. This almost looks like asphalt not concrete...or am I wrong? Asphalt will more easily come unglued than concrete. If you don't have a gas powered blower, you most certainly should think about getting yourself this wonderful tool! After mowing or spreading fertilizer you simply blow all your walks and drive way clean of debris and fertilizer. I couldn't live without mine!
One more thought, what are your street trees? This could be because of 'honey dew' falling from your trees. Honey dew is from aphids and other sucking insects and honey dew is like caviar to lots of other insects and plants.
Lichens are usually found anywhere that is damp, you mentioned trees, you can tell exactly how much sulphur is in the air by what species of lichen is growing there almost the exact (SO²)µg/m³ but I think the only option is to jet wash however looking by the condition of the surface- doing it too hard may cause damage- there are no chemicals that will get rid of it as lichen has a knack of becoming part of the surface their growing upon, the roots (if you can call them that) actually tunnel into the surface up to a depth of several millimetres and extract all their nutrients directly from the rock/atmosphere as lichens are a mix of fungus and algae- apart they can't survive but together form a new living species. Fertiliser and honeydew are not the causes- it just perfect conditions for the lichen to grow- in last few years air pollution that had sulphur in it has dropped thus things like blackspot and lichens have increased- the more furrier the lichen, the more pure the air is I would hazard a guess Dipoicia canescens?(125 SO²µg/m³ ) Lecanora chlarotera?(59 SO²µg/m³ ) or Palmelia caperata?(43 SO²µg/m³ )but there the only ones I know that look anything like the one on your photographs- hope it helps