What I have experienced (as many others as well) is that mild cases of Cryptocaryon with otherwise fit and healthy fish the infection may just go away without any intervention as the fish build up immunity.
The tests that have for sure been done are those regarding acquired immunity (Burgess 1992) and those findings are pretty conclusive: fish that have survived a nonlethal Cryptocaryon outbreak will acquire a level of immunity that it is proportional to the level of the survived infection.
To my knowledge not researched, but very plausible is that prolonged low level exposure can also lead to increases in immunity.
The single exposure tests (tests where fish were exposed to a single immunizing nonlethal infection) lead to full immunity after a recovery time of roughly 3 months when confronted with a lethal number of theronts. Microscopic inspections of those fish did not show a single trophont on the fish = the fish were fully immune. Fish that were exposed after 6 months only got a mild survivable infection. This however indicates that the acquired immunity is not permanent but would require regular exposure to the parasite to boost/maintain immunity.
Also, immune fish are actually toxic to the theronts, meaning any theront that tries to feed of an immune fish is actually killed off and not just deflected off to go and infect another fish with no immunity.
I had a clear case of full acquired immunity with a baby Maldives Regal Angel. In QT it survived a serious ich outbreak after hyposalinity treatment. When I transferred it later to my main system which still had a low level ich infection going at that time the fish never showed a single spot or symptom while all the other Regals there still sported a few spots once in a while (I put one of the Regals under the microscope to confirm that it was actually Cryptocaryon and it was).
Now, all this is not to be taken as advice to do nothing. If your fish have a serious and increasingly intense Cryptocaryon infection, a real treatment will be required.