What does a red skull above an army mean in Rome 2 Total War

Solution 1:

As mentioned by @5pike, the red skull symbolises attrition.

There are several reasons why an army or a fleet can suffer from attrition, here are some:

• Your army is currently laying siege (attrition applies after you end your turn)
• Your army is besieged by an enemy (attrition applies when your turn starts)
• Your army is currently suffering due to certain events (plague, sickness, earthquake, etc.)
• Your food surplus is negative and your army is currently in a province that imports food.
• Your army is currently in terrain it is not used to (e.g. desert, mountains) and not on a road.
• You cannot pay for your army's or fleet's upkeep (happens when a player has no settlements)
• Your fleet is currently in deep water

Solution 2:

The red skull means that they are suffering from attrition.

Sources:

Total War Forum

Penny Arcade Forum Post

Aside: Yeah, I know the sources are weak. I try to find a better one :P