Advance and be recognised.

       Friend of Foe?  Just like people, of whatever hue, not ALL Slugs are inherently GOOD/BAD.
      Behold:-
"These are Leopard slugs. They are a gardener’s friend. They don’t damage healthy, living plants, but they do eat other slugs, including species that can damage garden plants and vegetables. By eating dead and rotting plants, as well as fungi, Leopard Slugs recycle nutrients and fertilise the soil."
      BUT what it's doing half way the wall is a mystery - unless it's nibbling away at Algae.
HOWEVER!
      I read, farther down, that:-
"However, they can be confused with other slugs particularly this bad boy This is the yellow slug, although I would describe it as more yellowy olive green.
It is a large slug which can grow up to 10cm in length. The body is marbled and mottled in an olive colour. Its tentacles are blue. It is common and widespread in England, Wales and Ireland. It is mostly associated with houses and gardens – being known to venture indoors especially cellars after dark. Feeds voraciously on seedlings and vegetables so where it does occur on agriculture and horticulture land it can do serious damage."
      SO - must needs investigate further?