It depends how you look at it. The topic annoys me from a "strict" view because the number of marine PLANTS is very limited (certain sea grasses, mangroves).
But they are no doubt including algae and there are many interesting types though few end up in the hobby. In theory there could be a demand over time.
Halimeda and several other calcareous algaes are really cool (in particular I like Acetabularia crenulata)
From the Smithsonian:
There are really some absolutely beautiful alga but quite under appreciated, so we see Chaetomorpha, a couple of Caulerpa, and some nuisance stuff.
