Congratulations on your first brood of fry! A healthy interest in courtship and breeding is always a good sign.
If your tank has lots of live rock and supports an abundance of copepods and amphipods, your seahorses are almost certainly supplementing their daily feedings by grazing on the natural fodder in your tank. It’s very possible that this could account for a decrease in the amount of frozen Mysis they have been eating lately.
So, in your case, a decrease in their appetite may not necessarily be a cause for concern. As long as they are not losing weight or conditioning — no sunken abdomens or pinched in belly plates — but are maintaining their normal girth, then I wouldn’t worry too much about the change in their eating habits.
One way you can doublecheck to see if they are getting enough to eat on a daily basis is to examine their fecal pellets. If the fecal pellets they are producing have not changed in quantity or appearance, you can be assured they are getting plenty to eat. But beware if they start producing white, stringy feces rather than their usual fecal pellets — those pale, stringy feces are a sure sign of a seahorse that is not getting enough nourishment.
There’s really nothing you can do once pelagic seahorse fry gulp air and develop buoyancy problems. Once that happens, the fry are pretty much doomed. The best thing you can do is to prevent such problems from occurring by using kriesel-type nurseries with a circular flow pattern, maintaining reduced salinity in the nursery tank, and providing the right level of turbulence and turbidity, all of which will help keep the fry away from the surface and help prevent them from gulping air in the first place. If you haven’t already seen them, I would be happy to repost the information on nursery options for maintaining pelagic seahorse fry and preventing floaters and surface huggers.
Best of luck with your seahorses and their progeny, Seagazer!