Alfalfa is a legume, not a grass. It *is* a bit of a diuretic, but it is a good source of a wide variety of vitamins and minerals. I often add a bit of alfalfa (dried and powdered) to my homemade cat food (probably not more than 1/2 teaspoon to a few pounds of meat). Never noticed any problems from it.
Just for fun I looked it up on Google; the main site attacking alfalfa is, unfortunately, Feline Future. The article is so full of distortions and unfounded claims, it is really disturbing--since I have always considered them a good resource. Note that they were adding a full teaspoon of alfalfa powder per kg, which is a LOT.
Anyway, a very little bit is fine, but it is easy to overdo.