The northwest wall of the house receives sunlight filtered by trees and bushes and thus remains wet longer than the more southerly sides; it is a perfect habitat for cyanobacteria (blue-green algae). If it weren't for the resident slugs, a thick jungle of microscopic proportions would long have developed on the siding.
Slugs love the green stuff and on warm, humid days come out to graze on it. Here is an Arion subfuscus with its feeding track behind it.
There are a variety of tracks on the wall. Some are more elaborate than others. I suspect different species are involved.
This one reveals the marks of the individual teeth of the radula that does the scraping.
I am curious to know if I can tell the species apart from their feeding tracks. Updates will be posted.