When you are killing this mech boii he drops loot and green bag is falling through the ground. In the previous patch it was mentioned that this particular drop was only for one player when in fact it was falling through the ground and the player who was in the mech had more chances to grab it.
Well the green loot is mech part. Just to be clear, you have to be close to the Necramechs' body to have a chance to catch the loot. Itzal used to help with this since it's vacuum loot for everybody. But now you can't do anything. Pure luck.