Okay, that sounds like a very sensible plan. Go ahead and relocate your sensitive corals and inverts to another tank while you treat the seahorse tank with hyposalinity or osmotic shock therapy. Don’t let your specific gravity go any lower than 1.010 at any time! Electronic pH meters are more accurate and reliable than hydrometers, so you might want to make your target salinity 1.012 in order to provide you with a little margin for error.
Best of luck eradicating the Cryptocaryon from your system, Andrea!