I have heard "flying carpet" used to refer to swimming polyclad flatworms in general. Either way if you are referring to polyclad flatworms or nudibranches, they are sporadically available but virtually all have extremely specific diets. Most nudis are grazing predators on sessile invertebrates (commonly sponges, bryozoans, or hydroids). Polyclads vary a lot in diet but most are also grazing predators. Both polyclads and nudibranches are commonly monophagous - they have only one host (food) species. Also, 99% of nudibranches live only one year so it wouldn't be a longevid pet. Another problem with these swimming soft-bodied animals is they are easily sucked into powerheads and overflows and destroyed.
Kevin