Yeah it's aiptasia. Methods of removal include injecting with kalk
paste or lemon juice, basting with boiling water, or a commercial product like aiptasia-x or joe's juice. Injecting is a good choice. Aiptasia-X works pretty well too, they actually eat the stuff and die within a few minutes. If you're using kalk
or boiling water, you have to smother the whole thing so it dies. If you're injecting with lemon juice, you can take a hypodermic syringe and either inject through the mouth or the stalk. You can also buy a peppermint shrimp, many will eat aiptasia. You can also take a look at bhergia nudibranches, but that's overkill for a small tank.