You need to register marker click listener via GoogleMap.setOnMarkerClickListener()
and whenever some marker clicked - you need to remember this marker. That's the way to get currently focused marker.
Obviously you might want to "clear" last focused marker when user "unclicks" this marker (taps on a map, etc). In this case you need to register onInfoWindowClickListener
and onMapClickListener
and clear last focused item when these events occur