ulva, gracilera, caulerpa if you dont live in ca, red ogo, hypnea
Some
Caulerpa species are legal in California which tend to be easier to control (ex.
prolifera and
paspaloides). Other options are
Cladophora,
Enteromorpha (now under the
Ulva genus),
Codium,
Halimeda, Rhipocephalus,
Udotea,
Penicillus,
Botryocladia,
Bryothamnion,
Halymenia,
Haliptilon,
Coralline (encrusting, plating, and branching),
Dictyota, and Blue scroll. Note that none of these are true plants (while Mangroves and seagrasses are and may be added to an aquarium).
I have heard good things about Gulf Coast Ecosystems, ReefCleaners, eBay, Rusalty, Gulf Specimen Marine Laboratory, AlgaeBarn, Seahorse Savvy, and KP Aquatics as sources for Macroalgae and Mangroves. Seagrasses can be purchased at Gulf Specimen Marine Laboratory as well as Aquaticus Plants.
You can read up more on macroalgae on the Algae lab (AlgaeBarn’s blog), Marineplantbook.com, and the List of Marine Aquarium Plant Species on Wikipedia.