you can use chaeto algae, other macroalgaes. just do a bit of research on them and what people keep them with. brackish water is super easy. what makes these shrimp and amazing choice for the jar is that they are much more hardy than the toughest cherry shrimp you'll find. they can handle such a small aquatic system that you're trying to build.
With these, you don't have to do many water change for them. take a look at what other people do. they mostly just top off the water and maybe do a water change a few times a year. but you must cycle the jar as with any tank.
depending on the salinity you will use, some aquatic mosses can handle it.
Just consider the brackish option. you most likely will never regret it. plus its something new to learn about. you jar would be pretty unique