That little thing right there is the invincibility buff that certain enemies can have, preventing damage. It also looks like the Mithril Shield.
A nice addition to the game would be to have certain enemies have certain periods in time where if they’re hit, they get healed by the damage, rather than get hurt. The picture of the status effect being a Colossus Shield.
A potential problem with this would be trolls intentionally doing damage while an enemy is absorbing to waste time of everyone around them.