HTML Marker Icon

(under construction)

In your theme or in a mu-plugin:

add_filter('acf_osm_marker_html', function($html) {
    return '<div class="my-marker-inner">&times;</div>';
});
add_filter('acf_osm_marker_classname', function($html) {
    return 'my-marker';
});

Add some CSS:

.my-marker-inner {
  width:20px;
  height:20px;
  border-radius:50%;
  border:1px solid #f00;
}