We demonstrate the performance of the new code GR-Athena++ in evolving general relativistic magnetohydrodynamics (GRMHD) in a dynamically evolving spacetime. GR-Athena++ utilises the task-based parallelism and oct-tree based adaptive mesh refinement of the highly scaling Athena++ code, as well as its approach to solving GRMHD problems in stationary spacetimes; combined with new functionality to solve the Einstein equations in the Z4c formulation. We show the performance of this new code by simulating the evolution of Neutron Stars in a dynamical spacetime, presenting tests of our code, as well as strong and weak scaling tests.