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.