Thank you Oron I agree, it does fit the description of Milax barypus quite well, except that this species does not seem to be registered in Greece, and it is also smaller than those I see here. I looked at the description of all the 14 Milax species, some of them from Greece, but they were all smaller than those I found here (found more today, they are hiding under stones or in the ground). Then I looked at the species in the genus Tandonia (both Milax and Tandonia belong to the same family) at the site Terrestial slugs Web. The species in this genus seem to have the same size as those I have seen. I read that Tandonia sowerby is native to Europe, possibly originally only to Greece, where it is very common. The only deviation I could see is that the mucus produced by the sole of T. sowerby is a contrasting yellow , while the mucus I saw was clear. It seems to be a potentially serious threat, so I choose to treat it like I treat Arion vulgaris.