I have these on a live rock I added recently and fear aptasia, but the photos I see of aptasia show like I trunk with tenticles off the top. These are attached to rock in the middle and only the tentacles are extended off the rock.
Those are Aiptasia. A pest anemone it's best to remove them you can inject them with lemon juice, strait calcium, boiling water, joes juice, or aptasia-x all of which will kill them.
have always used aiptasia x and injected it into mouth area .in fact the tentacles grabbed needle and then i inject it always worked but found em again so got peppermint shrimp and never had em since
you can mix up some kalkwasser with a little water to a toothpaste consistency and inject it close to the center of the tantacles and it will dissolve like putting salt on a slug