ANY coral can bring in nasties. We once got flatworms in a trade. Lesson learned, dip corals. I hear Coral RX being recommended a lot but have never used it.
I've never actually thought about flatworms but i've got a banana wrasse, and if I do get them he should take care of them.
Been lucky with my tank had no bad things at all. Not even Aipitasia
I have heard good about coral RX also