Looks like an aptasia to me. I've been dealing with them for a few months now
. I have always used the aptasia x. Works great. I've also heard if you have one, you probobly have some you can't see.
Some of the other idea's i've been given.
A syringe with lemon juice injected directly in the mouth of the aptasia. You don't want to do this a lot since it will affect the water quality in large doses. You can only go after a few at a time with this technique.
and one person said they used the syringe to suck the aptasia out. no chemicals.
I've only personally used the aptasia x, so I don't know how well the other options work.