That's a great question. The Shedd Aquarium reported they never had a case of Cryptocaryon in their syngnathids. However, there is much speculation that it might occur in seahorses. I think I saw ich cysts in the gills of a dead seahorse under a microscope, but as I'm not a pathologist, I can't be sure that's what I saw.
Of all the disease a seahorse can get, that does seem to be low on the concern list because even if it is possible, it doesn't seem very prevalent.