What does the skull icon over an attacking Husk mean?

Solution 1:

Although this Reddit seems to be discussing a different kind of skull, someone also pointed out that the skull above enemies simply means that they have been marked:

I think your talking about a marked enemy. If you click the mouse wheel on pc you "mark" and enemy with a skull over it's head; it has no purpose other than to draw attention from your allies to that enemy.

This Reddit with a title of "PSA- You can "mark" enemies for other players to see, it will add a skull icon and is easily noticeable by everyone." also seems to confirm (they did it on Xbox One):

I'm a notorious panic knifer and my friend and I just found this out today by accident. On XboxOne you just click down on the right stick. It's pretty helpful for getting rid of the stronger enemies like smashers.