What does a white skull above the last enemy mean while boarding ships?
The white skull icon seems to indicate which enemy is engaging in combat with a crewmate that needs saving and is accompanied by a blue dot over said crewmate in trouble, this skull is to help indicate which enemy to focus on to help save that crewmate.
That means that one of your crew is in trouble! The white skull means that, that enemy is attacking your crew member, kill him before he kills your mate!