i haven't used reef dip, but the directions say it should only be used in a separate container not directly in the aquarium. if you are 100% confident they are only on a single coral or rock then you should be ok with just doing a dip, but my experience is if you have flatworms, they're not only on the coral you see, they're also other places in the tank... they like bright light and slow current. a good way to get them all in one place is to turn your pump and lights off and then shine a bright flashlight in one place on a rock for about an hour, they should all end up going to that one place and then start to siphon them.
you will probably like to treat the whole tank otherwise they will keep coming back. try flatworm Xit from sailfert. first you will have to siphon all the flatworms you can see out because when they die they release a toxic chemical. then after the treatment run carbon and do a water change. the sailfert flatworm Xit has very detailed instructions and i have used it successfully without any trouble.
the only thing flatworm Xit treats is the flatworms, although i have noticed my tiny brittle stars aren't to happy with it, but they did not die.