Yeah, you might want to confirm what you have before you go out buying livestock to kill it.
Try to get a pic posted so you can properly ID whatever it is you've got. Until then, can you answer these questions:
1.) Does the organism have a white tube/shell that the "plume" emits from?
2.) Does the actual organism look clear/transparent or opaque/not see-through
3.) Is the organism motile? I.e., has it moved or changed location at all?
4.) Does it have tentacles or does it look more like an inverted circular feather umbrella?
If it is feather dusters, I am sure someone will swap rocks with you as they are pretty good additions to increase biodiversity and filtration within the ecosystem. If not, then you can look into genocide techniques
Charlie once suggested I use "pickling lime" aka kalkwasser to kill aptasia spp. anemones and I have had pretty excellent results with that.
But yeah, try to get some pics posted or at least answer my questions.
ok, enough procrastination. back to the books for me...