If you can remove the rock from the DT and then remove the aptaisia IMO is the best method. Make sure you get every piece removed. If it retracts in a hole then seal it with glue. Injecting them with kalk can work or other aptaisia removers but you run a very high risk of causing a break out in the tank. Berghia nudibranchs work wonders as a natural predator. I've had success with peppermint shrimp and file fish (tomentous).