Yes, they are prone to ich. They have virtually no slime coat so no natural defense. They are very sensitive to copper, so that makes treating difficult.
I had an ostracion cubicus for 4 years, had ich numerous times but it never bothered him much. Until I introduced a fish that carried velvet. He got very sick so I put him in quarantine. He died sadly. Very soon after the water turned a milky white and started to have a very strange chemical smell. I am not sure what would have happened if I still had him in the display tank but I probably would have ended not well.
Still miss him, very personable fish but too risky for me.[/