I know nothing about touch tanks. I know something about macro algae and seagrass. Seagrasses need a sand bed 2-8 inches deep, depending on the species. Macros and seagrasses both require lighting as bright as reef lighting. Unlike reef tanks, planted tanks require lots of nutrients, so they have to be fed/dosed. I would imagine a horseshoe crab would tend to uproot seagrasses.
Can you describe your touch tank system? I'd like to know more. It sounds like a great idea! I'd like to help.