Mesclun is the proper term for a salad made of small, tender greens and lettuces-usually with about 3 weeks of growth. Often you see it listed on menus as Mesclun salad. Translated, it means "salad salad" kind of like using the term "Shrimp Scampi" which means "shrimp shrimp."
Mescaline is an hallucinogenic drug derived from the peyote cactus. If you see this spelling on a menu it is done in error or confusion.