Ghosts are easily distinguished by green color of their skulls and trails they leave behind. They charge directly at the Vault Hunters trying to self-destruct by smashing into them. This not only takes a portion of the target's shield but also inflicts the Terror debuff which negatively affects accuracy.
Vault Hunters may choose to outrun them as they will vanish after a short period of time. Ghosts are immune to cryo damage but because of their low health pool are not much of a threat.