Wow, I expected to see a very similar seahorse to the one in the first photo!
This second SH looks more like h. capensis, except that the coronet seems more pronounced. She isn't as spiny as the first, is she?
If she is capensis and the other is an erectus, you may have a couple of issues going on here.
1) If your pet store got these horses from a breeder, they would likely be the same species. They don't seem to be the same species, and this concerns me that they are wild caught (WC) and not captive bred (CB).
A WC horse is (MUCH) more difficult to keep because they are not trained to thawed mysis, and so you will have to feed newly hatched brine shrimp (preferably gut loaded) to give them nutrition. Brine shrimp (live or frozen) have no nutritional value once the yolk sac is depleted, after about 48 hours after hatching. This can be an exhausting way to have to feed seahorses.
You are doing your best by mixing thawed mysis with the live baby brine. You need to be sure they eat at least twice each day. If your tank has a copepod population, the horses will be able to at least hunt these for some nutrition.
If they are WC, they may eventually except the mysis as a food source, but it could be a long road.
You are probably doing your water quality a huge favor by removing the horses from the DT to feed them, but they might eat better (less stress) if you feed them where they live.
2) If these are the species I think they are, you have some temperature issues to think about. h. erectus is a tropical species, comfortable in a wider range of temp. The h. capensis is a sub-tropical species and needs a lower temp. If you can keep your tank at a lower temp (72-74f?). The most important thing is to keep the temperature STABLE, don't let it yo-yo.
Do these horses have tankmates?