Particles that appear when a 2x Rewards event is active.
2x Rewards
From the Hypixel Pit Wiki

2x Rewards is a minor event. When active, kills and assists within a randomly selected zone in The Pit (one of the quadrants, or the center) will grant 2× gold and XP. 2x Rewards lasts for 4 minutes.

If the current map is Genesis, the player will gain 2× faction points from kills in the active zone.

Green particles will appear while in the active zone.