Two or three years ago I found my first Leopard slug in my small garden in Kent, South East England, reading that that they were good for the garden, I did not kill it and since then the numbers have increased dramatically. Most nights I see four or five, varying in size from a centimetre or two up to 15 or more.
They tend to gather round fungi and especially like the Phallus Impudicus. If we throw out old catfood they will eat that and if I kill off other slugs I've seen them eating them. I've never ever seen them on a plant.
Are they unusual and I wonder why the numbers have increased so much?
I've attached a picture just in case its not a leopard slug. I notice that this is the only slug I've seen that does not shrink in length when touched, they are also fast movers and the biggest slugs I have ever seen!
The one in your photo may not be the best example (just google for images and you will see mostly spotted ones) but it looks like Limax maximus. They are omnivorous, so they won't just clean your garden of other slugs. Leopard slugs eat detritus (any leaf litter, compost bin in your garden?) and young plants. You can control slug populations in a small garden with parasitic nematodes that will infect their hosts with a deadly bacterium.