Strange Enderman drop

I provoked 2 Endermen in Hard mode (Survival) armed with a stone Sword. Whereas they would normally teleport behind the player, these Endermen didn't. Furthermore, once I had one down to half health, it dropped something which looked like a Purple Silverfish with no tail that was spinning and giving off the Endermen Particle effect. The remained even once then Enderman that had dropped it died.Both Endermen left one of them behind. If the blocks it was resting on were removed it would hover in mid-air nor was it effected by water. Attached is an image of the anomaly. The anomalies disappeared after I quit the game and returned.