Yeah, if all your other key levels are at 0, more than likely its the salinity. If I remember correctly from your previous posts, isn't yours a fairly new setup ... like not more than a couple of months old? If that's true, the red algae could be just new tank syndrome, which your tank will simply have to go through ... just keep an eye on it and make sure its not something that totally takes over your tank (if so there might be a more critical cause behind it).
One other thing ... besides the params you listed above, have you tested for phosphates and alkalinity? If so, what do those readings look like?
Have you done anything at all different in the recent past ... like add more rock, sand, etc? What kind of a star did you have? If it was a Blue Linckia star by any chance, I'd stay away from it ... they are extremely picky and sensitive to water conditions and are difficult to keep alive in a new setup.
Sorry to hear about the losses ... but try bringing down the salinity SLOWLY and see how things go. What other livestock do you have in the tank? How are they doing?
Regards,
Yash