First of all, it sounds like your seahorses are tank-bred/wild-caught. If this is the case, and you have not already done so, quarantine your seahorses for several weeks and de-worm them. Check for signs of malnourishment and common diseases in this time. It would be best if you could post some pics here so that more experienced eyes can check them for visible disease/malnourishment as well. Your tank sounds a bit overstocked, too. Make sure you have plenty of filtration and be diligent about water changes (15-20% weekly), ok? Seahorses are incredibly messy, so maintaining water quality is an important part of keeping them successfully.
Anyways, moving on to the issue of feeding your horses. A turkey baster with clear tubing is probably the best feeding tool there is. If you don't have one already, I would suggest getting one if at all possible. The baster can be used both to train your picky eaters to accept frozen mysis and to train those already eating frozen mysis to eat from a feeding station, which makes tank maintenance easier and also makes it easier to ensure that your seahorses are being fed sufficiently. In a 50g tank, sprinkling food all over is an inefficient method, and chances are that your seahorses are not getting as much food as they need. Especially if you are not feeding them 2-3x a day. So a feeding station is very important. As for how to train the horses, dangle a bit of frozen mysis from the tip of the baster and move it around a bit in front of them so that your picky eaters are tricked into thinking it is live. Do this before feeding them any live mysis. If they accept the frozen mysis, continue to feed them frozen mysis until they refuse it. To train horses to eat from a feeding station (which can be an upturned shell or perhaps a certain macroalgae/coral, basically just anything that will keep the food in place and provide a hitching post for the horses while feeding), do the dangling trick again, but this time use the dangling mysis to lure the seahorses over to the feeding station, which is filled with more mysis. It may take a while for the seahorses to get used to the feeding station, so be very patient with them.
Lastly, try culturing live mysis to feed to your seahorses while they are being trained to accept frozen ones. A google search should turn up many helpful articles on culturing live mysis. This way, you can stop worrying about how to feed your finicky seahorses and save money while you are at it. Furthermore, variety in food sources is important for seahorses, so it would be a good idea to get a hold of frozen brine shrimp to enrich and supplement their diet with, as well as live brine shrimp (decapisculated) to gutload and feed as treats. Larger species of Copepods and amphipods are also good choices for culturing. A variety of food sources will ensure that your seahorses have a balanced diet and remain healthy and well-fed. Investing in a small army of snails is another good idea. They will make a nice, seahorse-safe cleanup crew. Good luck!