>Is it possible this lichen grows on the slimy trails that slugs and such leave behind?<
Interesting suggestion. Bird droppings also provide a fertile ground for moss and lichen to grow. Houses with chimneys often have a lot of moss and lichen close to the stack, or along the ridge tiles, were birds are more likely to perch.
Snail tracks are pretty random but there may be other organisms that travel (or reproduce) in a more symmetrical pattern - not sure where this line of enquiry is taking us but it's certainly interesting.
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