Yes absolutely! They will love most macroalgae additions. We have therefore added an extra section for marine macroalgae which we recommend especially for a Seahorse aquarium. 
Macroalgae will not just provide for Seahorses beautiful natural holdfasts to keep them happy and healthy, but also improve the water quality by adding more oxygen and cleaning the water when growing

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Beside of this some macroalgae with a dense structure like the Ochtodes or Green Bush live algae species will offer as well great hiding spaces for copepods and Rotifers and Seahorses love to use this as a hunting ground :).

And we are sure you will enjoy it too, as it looks just so much more natural than using artificial solutions where they hold onto.

MACROALGAE FOR SEAHORSES