Slug Food is a necessary food that keeps slugs strong and healthy because a healthy slug is a happy slug and a healthy slug wins duels. It isn't supposed to be eaten by anything other than Slugs. It's a pellet-like food and variates from red to brown colours (passing by orange).
They can, of course, also eat normal food, but this is better for them.
When Slugs eat Slug Food, they often "personalise" it to their taste: