In A.B.Chandler's book Far Traveller, a collection of John Grimes shorts, he is a captain of a rich lady's yacht. They find a colony seemingly abandoned. The yacht, BTW, is sentient, and searches around and finds the colonists. When Grimes, the lady, and a party of robots get there, they discover the colonists all in a cave acting drunk. Cutting it short, the moss lining the cave provides all nutritional needs plus an intoxicant. The colonists have basically turned into vegetables, only eating the moss and having sex and sleeping.
The crisis for Grimes is that he falls into the trap of the moss, and it takes the lady quite some time and ingenuity to get him free of it.
Decent story, like most of the Grimes stories. Could fit easily into Traveller.