It is probably etched into the glass... If the haze is not salt, but micro scratches, then the only thing you can do is buff the glass with a fine wet sandpaper.
I had a problem with hazy glass on my tank and finally just got rid of it. I cut a piece of plastic egg crate material and used it for a lid. No etching, better gas exchange with the surface of the water, and my tank runs a lot cooler. I should have ditched the glass top years ago...