When Rafinesque erected the slug genus Philomycus, he explained that their name meant "friend of fungi". He had picked that name for them, because, Rafinesque wrote, they feed on fungi. (A reproduction Rafinesque's description is in this old post.)
The slug in this picture is Philomycus carolinianus, a native of eastern U.S. forests. This particular individual is living in a plastic box in my basement at the moment. It indeed likes to eat the mushrooms from my backyard as it was doing when I took its picture.
The smaller slug below it is an Arion intermedius, an introduced species from Europe.