Confidence: It's likely to be this, but I can't be certain.
Notes: Another very clear photo. In close-up, the skin looks translucent and without the opaque white glands that secrete the milky mucus of D. reticulatum. So I'd guess it's D. invadens (alias caruanae, alias pollonerae) which is also very common. Only slight doubt is the speckled mantle,whcih can occur in invadens, but is commoner in Marsh Slug, D. laeve. It looks rather big and chunky for laeve, but it might be a pale form of that. If it would stretch out more, the breathing pore is usually a large creamy patch in invadens, not in laeve. Turn it over, and the sole is pale fawn in invadens, usually darker brown in laeve.