hmm, no i dont know much about mandarins, do you have any macro algae in the display? Put some chunks of chaeto here and there in the display, you can hide it behind a rock or something, pods will reproduce in the middle and come out and get eaten. You can make pod piles from lots of small live rock rubble piled into a small hill, you can even culture your own pods outside of your tank, buy a bottle of live pods, put it in a 1-5 gallon container with saltwater at something around 1.025 SG, then put the pods in, and feed a few drops of phytoplankton in there, when you get a good amount feel free to put some in your tank every once and a while, i dunno if it would work but maybe mix the pods with some mysis to trick the mandarin or something.
With brine shrimp make sure you get some sort of enriched brine since adult brine isnt that nutritious, omega-3 enriched brine is easy to find. Baby brine is more nutritious than adult brine so you may want to get a hatchery going and feeding the mandarin freshly hatched baby brine. You could also get frozen baby brine but im never sure if that is really baby brine with egg yolks attached or not....