Cocoons can't attack the player, but they have alot of health and rarely spawn enemies.
Behavior
Cocoons just stay in one place, they don't move or attack the player in any way. Occasionally, an enemy will spawn from the cocoon, anything from a Normal Mutant to a Pre-Martian can spawn from a cocoon. It is advised to take out cocoons whenever they are spotted, so that enemies can't spawn from them; and to decrease the infection in an area.
Strategy
Cocoons have a lot of health, so it is best to attack them with a shotgun close up or attack them with a powerful melee weapon.