MyClusterManagerRenderer.java

    /**
     * Update the GPS coordinate of a ClusterItem
     * @param clusterMarker
     */
    public void setUpdateMarker(ClusterMarker clusterMarker) {
        Marker marker = getMarker(clusterMarker);
        if (marker != null) {
            marker.setPosition(clusterMarker.getPosition());
        }
    }

UserListFragment.java

private Handler mHandler = new Handler();
    private Runnable mRunnable;
    private static final int LOCATION_UPDATE_INTERVAL = 3000;

    private void startUserLocationsRunnable(){
        Log.d(TAG, "startUserLocationsRunnable: starting runnable for retrieving updated locations.");
        mHandler.postDelayed(mRunnable = new Runnable() {
            @Override
            public void run() {
                retrieveUserLocations();
                mHandler.postDelayed(mRunnable, LOCATION_UPDATE_INTERVAL);
            }
        }, LOCATION_UPDATE_INTERVAL);
    }
    
    private void stopLocationUpdates(){
        mHandler.removeCallbacks(mRunnable);
    }

    private void retrieveUserLocations(){
        Log.d(TAG, "retrieveUserLocations: retrieving location of all users in the chatroom.");

        try{
            for(final ClusterMarker clusterMarker: mClusterMarkers){

                DocumentReference userLocationRef = FirebaseFirestore.getInstance()
                        .collection(getString(R.string.collection_user_locations))
                        .document(clusterMarker.getUser().getUser_id());

                userLocationRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                        if(task.isSuccessful()){

                            final UserLocation updatedUserLocation = task.getResult().toObject(UserLocation.class);

                            // update the location
                            for (int i = 0; i < mClusterMarkers.size(); i++) {
                                try {
                                    if (mClusterMarkers.get(i).getUser().getUser_id().equals(updatedUserLocation.getUser().getUser_id())) {

                                        LatLng updatedLatLng = new LatLng(
                                                updatedUserLocation.getGeo_point().getLatitude(),
                                                updatedUserLocation.getGeo_point().getLongitude()
                                        );

                                        mClusterMarkers.get(i).setPosition(updatedLatLng);

                                    }


                                } catch (NullPointerException e) {
                                    Log.e(TAG, "retrieveUserLocations: NullPointerException: " + e.getMessage());
                                }
                            }
                        }
                    }
                });
            }
        }catch (IllegalStateException e){
            Log.e(TAG, "retrieveUserLocations: Fragment was destroyed during Firestore query. Ending query." + e.getMessage() );
        }

    }

thanks alot

THANK YOU SO MUCH