Is salt grass hay good for cattle?

Not if you're not prepared to pay for or pump out extra water for them. You will have to expect your cattle to be drinking more water than normal if you are going to be feeding them this stuff, as it would contain more salt than "normal" hay like Brome or Fescue hay. I believe that this hay is a bit coarser than most, so you will have to watch their body condition and how much and how often they are eating it to see how they're doing and whether it is good for them or not.