If you did not know, you should remove them as best and soon as you can. If left they will reproduce or whatever and totaly over-run your tank, killing all other corals (if you have any) during the process.
Here are some things that can help.. I got it from another website. Sorry, its a little lengthy...
Predators:
Thankfully there is a whole spectrum of animals that eat Glass anemones (Aiptasia), to some extent (what do you expect with such a bountiful natural supply?). These can/should be considered on the basis of their utility per your application, specificity of diet (some are errant nibblers on other sedentary invertebrates), and likely survivability (whether they live in captivity or no).
Elegance et al. Corals:
In the "who’s a better stinger" contest, the Meat/Elegance Coral (Catalaphyllia jardinei) rank near the top, higher even, than the Glass Anemones. If you could keep these mostly reef-system incompatible caryophyllids alive and were willing to risk moving them about to sting your Aiptasia (but not other animals), your pest control problems would be over. FWIW, Hydnophora corals are also better stingers than Aiptasia as well.
Nudibranchs,
Most popular is the little, almost invisible Berghia verrucicornis, are great obligate (this is all they eat) Aiptasia destroyers... do remember to "pass your specimen(s) on" a few weeks after your Glass Anemones are apparently gone. Otherwise the Berghia will perish from lack of food. NOT RECCOMENDED, VERY HARD TO KEEP DUE TO LACK OF FOOD!
Hermit Crabs;
Perhaps the best (cheapest, most readily available, easiest to track and remove...) Aiptasia nemesis are a few Hermit Crabs. In particular the more common "Red Legged ("Hairy") Hermit Crab, Dardanus megistos (Image) is an almost-all-the-time reef-safe animal that also eats pest algae. One or two to a tank is all it takes.
Shrimp, especially the Peppermint Shrimp, Lysmata wurdemanni (Image), can be a great Aiptasia muncher. Be aware that there is some confusion in the trade re this species and other Lysmata that are no good as designated Aiptasia munchers.
At right some of "the real thing" and a couple of "phonies": see Peppermint Shrimp to sort out who's who
Fishes:
Most celebrated is the Copperband Butterflyfish, Chelmon rostrata... now, if you can only find one in good shape to start with... and get it to not sample your living corals... A hardier choice in butterflies is the Raccoon, Chaetodon lunula... which generally will eat in captivity... but also is not above trying your corals, featherduster worms... If you use one of these fishes, you must keep your eyes on your other livestock. Other chaetodonts that have been cited as effective Aiptasia eaters include the Long-nosed Butterflyfishes (genus Forcipiger), Klein’s (Chaetodon kleinii), the Threadfin (C. auriga) and Tear Drop (C. unimaculata). Once again, be careful here... not all specimens will eat Aiptasia, and definitely not all will JUST eat Aiptasia.
Puffers of various sorts have made the Aiptasia hit lists of other authors. The Guinea Fowl (Arothron meleagris) gets HUGE, but I bet it would eat Aiptasia, along with everything else in your system. The Tobies, or Sharp-Nosed puffers, sub-family Canthigastrinae might be better, but they do would go roving for other food.
Some of the larger marine Angelfishes will eat Aiptasia for sure. The Queen (Holacanthus ciliaris) and some of the Dwarfs of the genus Centropyge have been cited.
In Europe, many aquarists utilize Scats (family Scatophagidae) to eat Aiptasia (and algae). These fishes are mostly sold as brackish aquarium animals in the west, and must be slowly (over a period of weeks) acclimated (back) to full saltwater concentration.
Chemical Means...
A few are touted, but these should be your option of last resort... most of these are outright biocides (killers of all living things), to non-effective... If you stoop to chemical means, be prepared to make large water changes to save your livestock from poisoning.
Strong Bases; caustic chemicals like Calcium or Sodium Hydroxide (the former is "kalkwasser", the latter the principal ingredient in oven cleaners) can be applied directly to the Aiptasia... in dry or concentrated liquid form... use gloves and forceps for the former, a sturdy pipette or hypodermic syringe for the latter... Given the choice, go with the Ca(OH)2.
Hot Sauce to the Great Unknown: Some folks are hawking other "novel" (this is a kind word!) materials to wipe out Glass Anemones, or make them go "dormant"... injected (good luck) to simply poured in. I’ve yet to see any credible evidence that these work on a consistent basis.
Other chemical approaches detailed in the literature include blasting the Aiptasia with concentrated salt mix, hot water, hot hydrogen peroxide... to dangerous chemicals like hydrochloric and acetic acids, copper compounds, even copper wire... don’t even think about using these.