I kept a mandy for 8 years that pretty much ate everything after awhile, however, in its last few months it got more picky.
A good food to get them eating is live blackworms, however, use them sparingly as they die quickly in SW. Live gutloaded Artemia is also a good food for them. You can also feed them live copepods, but be sure the pods you buy are demersal, such as Tisbe sp.. "Tigger" pods (Tigriopus californicus) are a swimming species which also don't reproduce well above above 68*F, so they're hard for mandies to catch.
HTH