A phrase commonly used in World of Warcraft to tell players to avoid spells (like poison clouds, rain of fire, tail sweep and similar) that often deals massive damage to players and thus can cause a wipe.
This phrase apply to all kinds of harmful spells, even if they're not coloured green or coloured at all.
The early boss Grobbulus in the raid-instance Naxxramas in WoW have the ability to put out green poison clouds under him. If the raidmembers stand in it they will die because of the damage.
The raidleader have been explaining the tactics and ends with: "And don't stand in green shit."