Plants and algae growing on mangroves, estuaries and (of course) anchihaline pools are highly succeptible tu grow well in our aquarii. Many reef tanks' algae are suitable, just try! Put your algae with original wa&ter in a plastic or glass jar and adapt it to needed salinity over several days or weeks then put it in your tank and see if it grow.
Vascular plants:Pandan, screw palm (Pandanus sp.): a palm like tree growing on beaches and salt marshes, only the roots should be submerged.
Mangrove tree (Rhizophora mangle): a tree growing in salt marshes, used in reef tanks as nitrate absorber, only the roots should be submerged.. May need to much nutrients to live in a small poorly inhabited Opae Ula tank.
Beaked tasselweed (Ruppia maritima): present in Opae Ula's natural environement, but protect in several countries, please no poaching
Green algae:Chaeto, supershrimp macroalgae (Chaetophora sp.): a common algae growing in reef tank, many species can be adapted to brackish water (and even freshwater in some cases), do it slowly or cells will explode. Available at Petshrimp.com
Supershrimp mossball/algaeball (Cladophorale spp.): unknown algae looking like a smaller version of freshwater marimo ball, may produce a lot of oxygen and buble enough to float. Available at Petshrimp.com
Marimo ball (Aegagropilla linnaei): regular marimo ball, slowly adapted to brackish water will grow and produce a lot of oxygen (enough to make it bubble and float) but large balls will break appart in smaller clumps or balls. A microscopic analysis is needed to verify if marimo ball and supershrimp mossball are the same or different species
Caulerpa (Caulerap sp.): several species of Caulerpa are found in estuaries and tolerate low level of salt, avoir light green individuals: they are already dead, take only opaque dark green ones. It can look like leaves, raisin or feathers.
Sea lettuce (Ulva sp.): common in reef tanks, this genera contain a lot of species, many are tolerant to brackish water and some wrow even is freshwater and tolerate both brackish and marine setup.
Brackish bladerwort (Batophora oerstedii): beware, this alaga seems big but its unicellular. The stem is nucleus free, it will die if you cut it. You need to take the root where nuclei are. BTW any other algae from Dasycladales order is suitable for brackish aquarium since all live mainly in salty marshes.
Red algae:Red moss (Caloglossa cf. beccari): rarely seen in the hobby, this red alga is sold as red moss or red liverwort for aquascaping. It grown on mangrove roots in brackish, fresh and full marine water.
Black beard/brush alga (Compsopogon sp.): a common pest algae in freshwater tanks but may form gay, pink, green or purple brushes.
Dichtya (Gracilaria ?): a common thalloïd red algae in french reef hobby, despite the latin/greek looking name, it is unidentified and may even be a popular name for several species (probably from Gracilaria genera). Try any red algae from a reef tank to see what happen.
Pulmon alga (Wrangelia penicillata): a white to pink feather like rodophyte.
??? (Bostrychia sp.): a world wide common algae growing mainly in brackish but sometimes in freshwater, on plants roots, forming thick dark red moss.
Also, its been asserted multiple times here that regular marimo balls simply can't handle the level of salinity in a supershrimp tank long term. If you have proof otherwise you should explain it in detail (I know you have said yours is healthy and growing, but do you have any other sources regarding the species handling high brackish salinities).
And as for the rest...unless you've been growing them and reproducing them for months and years I think it's a bit early to say "compatible with Opae Ula". So anyone reading this should do so with lots and lots of caution. There is a lot of misinformation already in Stalkers other thread ("Chaetomorpha can grow in freshwater", no...it cannot..."mustafa's algae balls are the same as marimo balls"...I've explained why I think it's unlikely at least three times and Stalker still makes this claim as if he's actually seen or owned my mossballs...not sure why..."all caulerpa can grow in brackish water"....do you know how many species of Caulerpa are out there...have you even tried a *single* species at low salinity over several generations?). That's my 2 cents. Proceed with caution. (I'm all for trying new plants in Supershrimp tanks, but I think they should be tested for quite some time before making absolute statements about their viability in Supershrimp tanks).