The LFS was probably talking about
artemia... A glorified (well, the correct scientific) term for brine shrimp. Typically they are not considered to be of very high nutritional value, although I have seen conflicting reports.
However, if you have a 6 line, I would advise against a mandarin. The six line does enough damage to a pod population on its own, and could make keeping a mandarin VERY difficult. If you are willing to spend the money on buying pods, or if the mandarin will eat the artemia (and maintain health and weight) then perhaps it is doable but probably not advisable...
Take er easy
Scott T.