If you've introduced any hard surface materials from another tank such as corals, rock or the like; reproductive cysts from the parasite can also be transfered into the tank.
If not, it's more likely not C. irritans
but rather a blemish of sorts that should clear up on it's own. I would do a few water changes with well aged SW
to reduce any DOC
. If the beginning of a small infection or bacteria problem, the water changes will help stem it's progression and allow the fish to do the rest.