Matted Filefish or Berghia Nudibranch. I had some a year ago and introduced Berghia and I have been clean since. It took about 3 months for all to be gone (time will depend on how many aptasia you have and the amount of Berghia you add). I did do a small experiment as I had a tank setup to culture more Berghia so I had people giving me their aptasia. I set some in a small tupperware container on my desk, no water changes, no food, and just day light and ambient light, they lived for 8 months before melting away.
You can also try Kalk paste, lemon juice or Joes Juice, personally if I used either of these I'd treat out of the tank.