What kind of Hawkfish? It might be "bullying" the Mandarin, keeping it from eating. It also might be out competing it, for the Pod population. If the Mandarin "suddenly" became thin, shortly after introducing the Hawkfish, it'd be worth keeping a close eye on the two, observing them, to see if there's any problems.
If you have a refugium, this will help to increase your "pod" population, by giving the pods a protected place to reproduce, without predation. However, adding bottled pods, to an already stocked tank, may not be of much use. The pods will quickly be consumed. Create a few "Pod Piles." These are small piles of pieces of rubble rock, in which the pods can hide, live, reproduce, that fish can't get into.
The pods you want the most of are Copepods, which are very tiny. The pods you saw with your flashlight, are most likely Amphipods, which are also great. However, I think Mandarins will make more use of Copepods.