Unfortunatly these things don't die or drop off the corals with any known dip other than fresh water, which will more than likely also kill the acro. They also take several months to grow into a decent size that MIGHT be seen after the coral has been left out to "dry" for some time.
As you can see in my pic, they are very small and transparent after hatching from the eggs. It take about 3 months for these to reach a decent size. So if eggs went unnoticed in a system. The eggs hatch, new very small, clear flatworms. No treatment will remove them without the probable death of the acro. How can we tell?
About the only thing we can do is know what to look for. Tissue recession, usualy from the base up. These things for some reason almost always start at the base of the coral and work their way upwards. They even tend to hide in the folds of higher branches during the day, then move down to eat and lay eggs at night. About all we can do to be 100% safe is QT our new acros for 3 months!! 3 months!! That means a dedicated qt system for a 3 month period. No other acros introduced to this system for that time period.
These things are much different than red mites. If you place a red mite infected coral in your tank, they spread every where. These are for the most part coral specific. EX: Of the corals I picked up, 4 millis and 2 nana/valida type acros. 2 of the 4 millis were infected, I fraged the other 2 and dipped the large or main portion in fresh water and found nothing. Of the 2 valida/nana the Larry Jackson was completely covered, at least 50 dead flatworms in a 1 minute fresh water dip. The very similar steel blue acro, sitting right next to the LJ, nothing. And I ripped this coral apart. The steel blue was a nice little 3" colony. I broke it at the base, kept a small frag from the top of the coral (incase it was clean) took the mini colony ALL of it, and dipped in fresh water for about 5 minutes. Not one flatworm. Why? No one knows yet. No one knows what makes them stay on a coral or leave it. They do have corals (like the LJ) that they seem to prefer, oddly millis normaly are not one of the prefered corals, but I believe that the millis are where the problem started. In another system, other more prefered corals were already attacked and they moved on to the millis that ended up in James tank...then my qt. ...I have to go to work for a couple hours. I have a couple ideas I'd like to run by all you fine, fine lady's and gentleman