Originally Posted by steve-s
Polyclads are carnivors and not considered harmless at all. Most have specific dietary needs and what they eat will depend on the species you have. If it looks something like Paraplanocera oligoglena
then possibley an polyclad. A better photo would help.
I agree they are carnivores. As I understand it, most are not harmful to corals but may eat clams, tunicates and bryozoans etc. But, I've been wrong before. This is why I said "most are harmless" and to "keep an eye on it just as a precaution". I never removed my larger flatworms and they never bothered a thing in my reef that was noticable. I haven't seen them in over a year so that would tell me they died out on thier own. Shame really since they are interesting to watch.
Bob Fenner's site says "All polyclad Flatworms are carnivorous, and specific about their prey. Most feed on sea squirts, bryozoans, small worms, crustaceans, or snails, but some are cannibalistic"
Seaslug forum says pretty much the same thing. (though this is the only place I've seen mention of eating corals
These quotes I got from an article from Dr Ron Shimek.
"The free-living flatworms are a diverse group and we can expect to find some in our aquaria from time to time, often introduced on live rock or from some other such source, although most donâ€™t live for long periods in our systems."
"Relatively little is known about the natural history of most free-living marine flatworms, and specifically the tropical forms are more poorly known than the temperate forms. Many, such as Pseudoceros and Kaburakia, are known to be predatory. Species in the former genus often eat sessile invertebrates, such as tunicates, while Kaburakia excelsans appears to feed on mussels and barnacles. The smaller acoels are often considered to be herbivorous, but there is anecdotal evidence that at least some of them are predatory on protozoans and invertebrate larvae."
Being specific doesn't necessarily scream "threat" to me but not everyone's preception of that is the same. I'd only be concerned if there were ornamental clams in the tank (maxima, crocea, etc), and then only if you see the worm on the clam. Most of these other food items I've seen listed you never see anyway. Innocent til proven guilty is my theory but if you feel like it's to much a risk, then by all means, remove it.