Anthraxx is correct.
You can try lemon juice. Or there is a product called Aiptasia X. It comes with a syringe applicator. It works very well on aiptasia. You do not want to get it on your other corals.
If you want to try lemon juice, go to a pharmacy and buy a couple needles. They sell them for around 25cents each.
You can also get a peppermint shrimp. Most will eat them. There are also bergia nudibranches.
The feather duster in the 3rd pic is a good thing.