Here's why "reef safe" and "antiparasitic" don't often coexist. Think about the animals you have in your tank: corals and fish, of course, but also mollusks, other invertebrates, dinoflagellates, bacteria, viruses...a lot of organisms with different lifestyles and mechanisms, and all of them playing a vital role in the ecosystem you have created.
When you treat a human being who has a bacterial infection, the way to kill the bug and not the human is to choose something the bacteria does metabolically that humans do not, and target that. So, we can make a medicine that attacks the cell wall of the bacteria, because we don't have cell walls. Bacteria are affected, we are not, bacteria die, all is good. When we move into parasites like tapeworms and roundworms, or mosquitoes, the same idea applies: pick something the parasite needs that we dont, and attack that.
Now, take the reef tank with ich and apply the same principle. You want to find something that attacks the Cryptocaryon, but doesn't attack anything else, so what does Cryptocaryon do that nothing else does? And remember, Cryptocaryon is a dinoflagellate. What else is a dinoflagellate? Zooxanthellae. So you would have to find something that affects one dino with out affecting the other...and you can start to see why this is a challenge.
There are games you can play in medicine where you can try to use a treatment that will hurt everything, but will hurt the bad thing faster, but that's risky, and certainly not something I would consider "proven safe".