it wont get big. you will just get them all over your tank! get a peppermint shrimp. it will eat them, plus they are good for the tank! or get joe's juice. or aptasia remover. it's a syrange that you spray medicine on the aptasia to kill it. or you can use a small ear wax syrange with boiling water and burn it to death!
eather way, kill it asap! it will reproduce like dandolions in your yard!