this is gunna sound crazy, but...take the rock out and blast the aiptasia with one of those barbeque or stove lighters, the ones with the goosenecks on them. Flame it for about 30 seconds, it will be completely done for. Then rinse the rock in fresh water for a couple pf seconds. If you really need to, you can go over the spot with a bristle brush, old toothbrush, or even a screwdriver or a razor. All gone.