Symptom

On mouse hover, a blue polygon will show up behind a grouped map (cluster) marker within a map. 

Explanation

This is standard for when site visitors view the map as it indicates where the Map Locations are within that cluster.

Resolution

For instances in which this is preferred hidden, you can try to use CSS to remove the background shape. However, this is unsupported since this is fixed within the plugin. It may or may work but the below class may be explored: 

leaflet-overlay-pane.leaflet-zoom-animated