I guess you are using the Leaflet.markercluster plugin, and are asking how to customize the coverage polygon that is displayed when hovering a cluster, or how to disable this feature?
In that case:
showCoverageOnHover option:L.markerClusterGroup({
showCoverageOnHover: false
});
polygonOptions option, with an object containing appropriate path style options:L.markerClusterGroup({
polygonOptions: {
color: "red"
}
});