[rayjay]

I'm not sold on them being WC, especially as there are fewer places now selling WC when the availability of raised seahorses is more prevalent.
While moving stress CAN impact their eating for a while, it sounds like you introduced them to an already existing tank which may have copepods/amphipods in there that the seahorses may be getting and you not seeing this. Even knowing this though, it IS now critical that you see them eat SOMETHING, as if they go for long without, they may never start to feed again and would need tube feeding.
As these have produced fry AND eggs, we know that they are big enough to have become sexually mature, and MANY seahorses that size would have NO interest in brine shrimp nauplii. Also, even if they DID eat them, it would take an inordinate amount to be of any value nutrition wise.
The ghost shrimp you introduced may be too large to interest a juvenile even though it's large enough to be sexually mature, but without seeing them I can't say for sure.
Can you access adult live brine, or can you get pods that are amphipods or similar size to try? If using ghost shrimp can you get ones that are similar in size to adult brine?
Worst case scenario is that the seahorses have been in store systems that are connected to other fish and livestock where they may have picked up pathogens they haven't grown up with and have little resistance to ward off.
Seahorse immune systems are probably on average, the WORST of any marine fish we see normally in the hobby.
As a last resort, if feeding does NOT appear to happen, then I'd try a 12 minute fresh water dip to see if gill parasites are possibly causing the seahorses not to eat. If so, when placed into the dip with water matching temperature and pH of the tank water, severe thrashing would occur, AND you would SEE the tiny parasites come off into the water as their bodies explode on contact with the fresh water. If NO thrashing is observed the likelihood of this being the cause is slim to none.