Berghia nudibranchs are so effective at destroying aiptasia (without harming anything else in your tank) precisely because the only thing they can eat is aiptasia. That means that once the aiptasia in your tank is gone, the berghia will die off. However, as they get hungry, the berghia will have a tendency to "come out" and attach to the tank glass. You may be able to rehome them at that point.