Yes, sir, newborn seahorses will also be rotifers, copepods and larval Mysis, among other things, in addition to newly-hatched brine shrimp. However, it is much more difficult and much more expensive to provide these alternative live foods in the quantities necessary to raise seahorse fry, Artemia nauplii (newly-hatched brine shrimp) are the most practical live food for the home hobbyist to provide.
Some breeders report limited success rearing seahorse fry on nonliving foods such as freeze-dried or frozen Cyclop-Eze or preserved zooplankton, but the vast majority of newborns will not except such offerings, at least initially, and losses are typically very, very high for those who have tried such shortcuts. The non-living food simply doesn’t move right, and usually fails to elicit a feeding response from the fry.
Frozen Cyclop-eeze and preserved zooplankton can be great time savers when rearing fry but they do have one other very big drawback. They are not feed-and-forget fry foods like Artemia nauplii or live copepods which survive until eaten and keep swimming around until they pass within striking distance of one of the babies. If the preserved zooplankton isn’t eaten quickly, it will settle out of the water column and begin to degrade the water quality in the nursery. So if you are relying on Cyclop-eeze and preserved zooplankton to make up a large portion of your frys’ diet, you need to be even more diligent about making water changes and siphoning off the bottom of the nurseries in order to stay on top of the water quality. You may need to increase the frequency of the water changes and/or increase the percentage of water you change each time, if you try such foods.
Best of luck treating your fry, Nigel!