I don't think it will kill them, but ive heard it can stunt their growth. Someone more experienced with salt would be able to tell you better, but from my experience personally, it doesn't really harm them. I couldn't say if it stunts them, because i had plants in my quarintine tank with salt added, but the plants soon went in the garbage, as i had too many. So i don't know what the long term effects are.
My guess is though, if you have something like crypts, they would probably melt. Just assuming though, and again, im not sure...........