add_filter('wpgmp_map_markercluster',change_cluster_marker,10,2);
function change_cluster_marker($cluster_data,$map){

// Add map specific condition here

if($map->map_id == '1'){

//Change Main Cluster Image
$cluster_data['icon'] = 'university.png';

//Change Cluster Hover Image
$cluster_data['hover_icon'] = 'university.png';

}

return $cluster_data;

}