It is fun to have mangroves and sea grass. We see a lot of these along the shore of the bay after storms. Black Mangroves and Sea Grass are protected species so we can't just go unroof it from the wild and plant them in our tank, but collected unroofed grass and seeds along the rocky sea shore, technically against the law, I collect them anyway and replant plant them anyway in my tank.
How much nutrient they remove? Likely minimal. I doubt that they remove anywhere near the macroalgae remove. I remove about 1 cubic feet of algae from my tank every week or two, I also remove more than a gallon algae (compressed) from my 65 gal weekly. The Sea Grass and Mangroves just do not grow fast enough.
Weekly from 65 gal. Dimension of the container is 18 inches X 8 inches