You see a brain parasite
Notes
It's part of the fight against The Brainstealer boss of the Too Hot to Handle Quest.
Abilities
Melee (0-300)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Behaviour
A brain parasite never retreats.
Strategy
Ignore them or kill them, but they'll respawn in the room so focus on the boss and the mechanics.