Personally, I'd go with some calcified macroalgae, as - to my understanding - most herbivorous fish find these unpalatable and will leave them alone (things like mermaid's fan algae, shaving brush algae, pinecone algae, certain Caulerpa spp., Halimeda spp. etc.). Alternatively, if you do want to use mangroves, you can fill a little container with sand/mud and use it as a pot/planter for the mangrove in the tank - that way you wouldn't need a deeper sand bed in the tank, as the tree would have its own deep sand bed.
That said, there are likely a few other plants that could grow out of the water in marine conditions (these plants are known as hydro halophytes or aqua-halines), but, to my knowledge, the tropical water ones of these are typically more estuarine/intertidal species (so you may need the water level to rise and fall daily to prevent killing the plant, and you may lower salinities for many of them). If you were running a coldwater tank, you may have some better options, but I haven't looked too deeply into this yet. Either way, with real plants you'd need deeper sand/mud for their roots, which means either a deep sand bed or a planter like I described above.
