Generally unrelated, but there’s a type of algae (acetabularia) that looks like it should be multicellular, its a macroscopic “plant” with a clear if simple structure, with roots, a stem, and a mushroom cap looking plate on top. But, its actually a single celled organism, and unlike many other single celled organisms that are big enough to see, which often have multiple cell nuclei and effectively act like multiple cells just without the walls separating them, it only has one nucleus as well. Algae are weird.