Invisible Mobs

When i was playing with my friends in Survival Server, there are invisible mobs that we don't even know what it is, yes i use addons but i don't even think it affected the mobs either, the invisible mobs cause our server very laggy and cause fps drops. I was searching the Internet for the same issue but it was none.  i attached the video of my friend playing, even when he use gamemode creative and put lava in the floor and yes, the invisible mobs can't even die with lava.