Thanks ladies!
Yes, it was eating the mushroom, @Shirley_Freeman - I’ve seen slugs do it on corals and other kinds, but not a snail before so this was new to me.
Those are its eyes actually, @Bonnie_Lampley - and one is completely foreshortened because it’s pointing right at the lens. In some shots from the other angle I could see it move from shot to shot as the snail pulled it in. Such strange critters.