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.

Yeah I looked it up before posting, and although there are many common types of green algae, (many that are not single celled) the genus Desmodesmus or often clasified as Scenedesmus algae is said to be the single celled version here.

https://en.wikipedia.org/wiki/Valonia_ventricosa

one of the largest known unicellular organisms!