One possible strategy to consider is outlined below:

To enhance the functionality of ClusterIcon, a new event called clustermouseover can be introduced and triggered on the mouseover event:

//Please note that the code snippet provided here is just a part of the entire function
ClusterIcon.prototype.onAdd = function() {
    this.div_ = document.createElement('DIV');

    var panes = this.getPanes();
    panes.overlayMouseTarget.appendChild(this.div_);

    var that = this;

    google.maps.event.addDomListener(this.div_, 'mouseover', function() {
        that.triggerClusterMouseOver();
    });

};

In this context,

ClusterIcon.prototype.triggerClusterMouseOver = function () {
    var markerClusterer = this.cluster_.getMarkerClusterer();
    google.maps.event.trigger(markerClusterer, 'clustermouseover', this.cluster_);
};

An event handler can then be attached for displaying relevant information. The following code snippet showcases how to display a list of names:

google.maps.event.addListener(markerClusterer, 'clustermouseover', function(clusterer) {
    var markers = clusterer.getMarkers();

    markers.forEach(function(marker){
        infowindow.content += '<div>' + marker.title + '</div>';
    });
    infowindow.setPosition(clusterer.getCenter());
    infowindow.open(clusterer.getMap());
});

For an example implementation, you can check out this Plunker

Here is a method that has worked for me in initializing a map:

public void initializeMap() {
            googleMap = mFragment.getMap();
            googleMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
            googleMap.getUiSettings().setZoomControlsEnabled(true`enter code here`); 
            googleMap.getUiSettings().setZoomGesturesEnabled(true);
            googleMap.getUiSettings().setCompassEnabled(true);
            googleMap.getUiSettings().setMyLocationButtonEnabled(true);
            googleMap.getUiSettings().setRotateGesturesEnabled(true);
            if (googleMap == null) {
                Toast.makeText(getActivity(), "Sorry! unable to create maps",
                        Toast.LENGTH_SHORT).show();
            }
            mClusterManager = new ClusterManager<MyItem>(getActivity(),   googleMap );
//          googleMap.setInfoWindowAdapter(new CustomInfoWindowAdapter());
            googleMap.setOnMapLoadedCallback(this);
            googleMap.setMyLocationEnabled(true);
            googleMap.setBuildingsEnabled(true);
            googleMap.getUiSettings().setTiltGesturesEnabled(true);

MyItem offsetItem = new MyItem(Double.parseDouble(outletList.get(i).getMap_latitude()),
                                           Double.parseDouble(outletList.get(i).getMap_longitude()), title , address);
            mClusterManager.addItem(offsetItem);
            googleMap.setInfoWindowAdapter(new CustomInfoWindowAdapter(offsetItem));

}


    private class CustomInfoWindowAdapter implements InfoWindowAdapter {
        Marker marker;
        private View view;
        private MyItem items;

        public CustomInfoWindowAdapter(MyItem item) {
            view = getActivity().getLayoutInflater().inflate(
                    R.layout.custom_info_window, null);
            this.items = item;
        }

        @Override
        public View getInfoContents(Marker marker) {

            if (marker != null && marker.isInfoWindowShown()) {
                marker.hideInfoWindow();
                marker.showInfoWindow();
            }
            return null;
        }

        @Override
        public View getInfoWindow(final Marker marker) {
            this.marker = marker;

            String url = null;

            if (marker.getId() != null && markers != null && markers.size() > 0) {
                if (markers.get(marker.getId()) != null
                        && markers.get(marker.getId()) != null) {
                    url = markers.get(marker.getId());
                }
            }

            final ImageView image = ((ImageView) view.findViewById(R.id.badge));

            if (url != null && !url.equalsIgnoreCase("null")
                    && !url.equalsIgnoreCase("")) {
                imageLoader.displayImage(url, image, options,
                        new SimpleImageLoadingListener() {
                            @Override
                            public void onLoadingComplete(String imageUri,
                                    View view, Bitmap loadedImage) {
                                super.onLoadingComplete(imageUri, view,
                                        loadedImage);
                                getInfoContents(marker);
                            }
                        });
            } else {
                image.setImageResource(R.drawable.ic_launcher);
            }

            final String title = items.getTitle();
            Log.e(TAG, "TITLE : "+title);
            final TextView titleUi = ((TextView) view.findViewById(R.id.title));
            if (title != null) {
                titleUi.setText(title);
            } else {
                titleUi.setText("");
            }

            final String address = items.getAddress();
            final TextView snippetUi = ((TextView) view
                    .findViewById(R.id.snippet));
            if (address != null) {
                snippetUi.setText(address);
            } else {
                snippetUi.setText("");
            }