Yes, you’re going away for an extended period, underfeeding is vastly preferable to overfeeding. Feeding once a day is a very good option, but unless your fish sitter is an experienced aquarist who is familiar with seahorses, I suggest feeding the seahorses with live foods every second or third day instead.
When you’re going to be away for a week or two at a time, I would recommend ordering some hardy live feeder shrimp, some of which can safely be added to the tank every couple days, knowing they will survive in the aquarium until eaten. Ocean Rider’s red feeder shrimp (Red Iron Horse Feed, Halocaridina rubra) are ideal for this. They are what I’d like to call a "feed-and-forget" food. They are tough, rugged little shrimp that you can toss in your tank with no acclimation whatsoever. They are agile and elusive enough that your filters won’t eat them and the seahorses won’t be able to capture them all right away. Some will hide and evade well enough that your seahorses will still be hunting down the stragglers for the next day or two. Best of all, you can toss a batch of them in your aquarium, secure in the knowledge that they won’t perish and pollute it, but thrive and survive as real, live, "catch-me-if-you-can" prey items that seahorses cannot resist. Nothing stimulates a seahorse’s feeding instinct like the frantic movements and evasive maneuvers of natural, living prey.
So if you are planning on being away for an extended period of time, Dawn, I would suggest ordering a 100-500 count of Red Iron Horse Feed in advance, and setting them up in a small tank of their own with a small algae-covered live rock before you leave. That way, when your "fish sitter" checks in on your aquarium every so often, he can just add another netful of red feeder shrimp to the tank and that should take care of your seahorses’ feeding requirements until he stops by again.
Aside from adding the live shrimp, your fish sitter’s duties will be extremely simple, mainly just checking to see that everything is operating properly. Power outages, equipment failures, or the untimely death of a specimen can wipe out your tank if they happen while you’re away. Consider recruiting a reliable coworker or trustworthy neighbor to look in on your tank periodically while you’re gone. They needn’t be aquarium savvy at all, since you will be assigning them only the simplest of tasks: (1) check to see if the equipment is on and operating properly; (2) add freshwater to replace water lost via evaporation; and (3) toss in a netful of live red feeder shrimp. And that’s all. In fact, it’s a good idea to forbid them to touch anything or do anything other than those 3 very basic duties. And, of course, you will be familiarizing them with your setup beforehand, leaving them a plastic gallon jug of dechlorinated tapwater or RO/DI water with which to top off the tank, and providing a supply of red feeder and a net so all they have to do is scoop some of the shrimp and dump them in the tank. (Stick with the live food if you recruit a fish sitter, Dawn. That way they won’t have to deal with preparing frozen mysis and there’s no danger they’ll overfeed it. I learned the hard way that inexperienced seahorse keepers ALWAYS have a tendency to grossly overfeed, but that’s not a concern with red feeder shrimp.)
That’s how most seahorse keepers cope with trips and vacations, and in my experience, with a little advance planning it generally goes quite well.
Best of luck with your new seahorse setup, Dawn!