Ah I see! I used it on the valve cover threads so it shouldn't be a problem.being a spring filling in excess space there is nothing holding it into place.
as you torque spark plug the helicoil will deform the threads cut into the aluminum slightly each time plug changed.
This won't cause problems at first, but as years go by and the number of times plug changed helicoil can loosen enough to extend into cylinder when a new one installed or retract itself when plug removed..
As you have already installed helicoil I would use anti-seize compound on spark plug threads and be extremely careful not to over torque..