Good deal! That’s encouraging that your skinny Hippocampus reidi is pigging out on the frozen Mysis. You’re right — it’s a blast watching the seahorses go hunting for live prey and stalk the shrimp.
The live Mysis aren’t as easy to keep for long periods as the red volcano shrimp (Halocaridina rubra), so I might be inclined to dole them out to your H. reidi more frequently than once a week, just in case you start losing some of the live Mysis in your holding tank. Maybe offer them a few times a week as a compromise — gradually enough to make them last for a while but often enough to give you a better chance of going through the whole branch before they die off for one reason or another.
Also, if you want to try culturing live foods for your seahorses at some point, there are a lot of good options you can consider besides the live Mysis or the volcano shrimp. Let me know if you’re going to give that a go and I’ll be happy to outline some of the other live foods that would be good candidates for such a project.
Best wishes with all your fishes, Erik!