My shrimp spend their time on all kinds of moss. Moss balls maybe a bit less than other kinds. If you get a moss ball, give it a squeeze out in clean water now and then, as it will get full of dirty water and detritus inside. My scuds like the moss balls and wriggle into them to eat the detritus inside. It may float for awhile after this, but will sink again. Or squeeze it under water to remove the trapped air that causes it to float.
If you have space and can afford them, get both kinds. If not, get Java, or Christmas or Flame. All three of these mosses work well, and are about the same in terms of growing them. Flame or Christmas are more attractive than Java is.
Even Phoenix moss [ fissidens fontanus] works but is so slow to grow, you won't get any great size of it without a lot of time. Don't get willow moss, it's pretty hard to grow.
Mini pellia, which is a miniature liverwort, can also work, if you find some. But it really has to be contained until it has grown a fair bit. Netting works best to hold it snugly to a rock until it begins to take hold. It'll grow, both to hold onto the rock and through the netting, which will eventually be covered so the net can't be seen.
You can do the same with mosses, to attach them to wood or rocks. Thread or fish line will work also to tie lengths of java and other mosses to their mounts. One cheap source of soft netting would be dollar store bath puff. They're most often made of two lengths of simple, soft plastic mesh. Just cut the cord that ties the bundle together. You can cut the mesh to any size needed.
I've started giving my shrimp small Vacation style fish feeder blocks too. I used to give them just to the snails, but I've noticed shrimp picking at them whenever the blocks were there. Now I give them to shrimp all the time and they pick at them constantly. The blocks I used are 'nano' sized, with spirulina and fish food in them. You could make them yourself if you're into that kind of thing.
The blocks are made of Plaster of Paris, which is pure calcium sulfate, beneficial to shrimp and snails alike. Just mix it up as directed, adding some fish flake, pellets or alage powder, then mould it. I have some mini ice cube trays that do great for moulds. Let the mix set up and then turn out of the mould to cure fully. They won't cloud the water,, just replace when they're gone. I've been surprised how quickly they disappear.
