I have never seen one of these before - looking it up it is a leaf veined native slug. I was removing the leaves? of the yukka and they are on the underside of one of the leaves.
The coin is a New Zealand dollar - 2 cm across. In the second photo one of them has gone to the underside of the leaf (there were three in the top photo. I will take the leaf down to the gully and they can roam there.