Lemon juice, white vinegar. Both works and uses the principal of having a much lower PH that actually dissolves they aiptasia.
For that to work, you need to ensure that the lemon juice or vinegar do not wash away. Do not apply to fast and its gone. Do it slowly and use an insulin needle. So slow that you can actually see the pests turn grayish white as it dissolves. Focus on the foot area.
Best results, take the rock out an put it in a container with old tank water. Let it be until it reaches out. It will stretch out a lot more due to no currents.
Thanks @RiaanP. Unfortunately it's going to be difficult to remove the rock. I will have to do this in place.
Also, do I actually inject the foot with lemon juice or do I apply the lemon juice to the outside of the foot using the needle for better control?
You will have to redo this again, as they do tend to spawn when attacked. If they are matured. Babies will pop up. The guys from Red Sea had a presentation long ago at Pet stop. When asked why their Aiptasia-X does not work, he replied that you need to redo the process before the babies reach maturity. He mentioned that the next killing round must be before 43 days.