Macro in a seahorse tank is quite common and a good idea for several reasons. It provides a hitching post for the seahorses, it absorbs nutrients, seahorse's product a good amount of waste. Years ago, I had a very successful seahorse tank the consisted of live rock and Caulerpa racemosa. My seahorses thrived and my water parameters were on point. When you harvest the macro, you can feed to your reef tank provided you have algae eaters. I would caution putting some macros in a reef tank unless you have animals that will eat the macro, or you are trying to go for a macro reef tank. Caulerpa prolifera, and racemosa are two that I can think of that if you get in your tank are difficult to get out and people regret putting them in their reef tanks.