I have never used the Reef Bugs but Mark Weiss is a reliable source for several interesting products, and I can see how they could be a viable food source for filter-feeding invertebrates. As long as you use them as directed, I don’t see how they could cause any harm in your seahorse tank. Microbes and microorganisms are too small for seahorses to feed on directly, but the uneaten microbes will colonize the substrate and may serve a useful role as detritivores, if nothing more. It sounds like Reef Bugs might be an excellent product for helping live sponges to thrive, which would make them useful for seahorse keepers with colorful tree sponges in their tanks.
Best wishes with all your fishes (and invertebrates), David! Please keep us informed regarding how well The Reef Bugs do in your seahorse setup and whether or not the seahorses have any interest in them.