Will most likely be extremely stressful for the corals. I have had success doing that in the past, but the corals took a long time to recover. They will lose most tissue, and can look almost dead. Would not recommend anything longer than 1min, jme.
Might get Flatworm exit and make a dip solution with it and some tank water. You could probably make it a little strong to help weaken the worms enough to get them off the coral.
Melafix is your best friend against flatworms, 4 to 5 minute dip, rinse and QT if possible? I've done a RO dip/rinse when dipping new frags in the past, with no residual side effects. Especially if I suspected something was on the frag or plug.
Freshwater dipping is super stressful on sps and I have lost frags in the past doing so. Best case was they browned out, looked nearly dead, no PE and flesh looked very thin for several weeks. There are several effective dips for removing flatworms even AEFW.