Protoplasm is a creature exclusive to the show, Captain N: The Game Master, seen onboard the Metroid asteroid in the episode, Metroid, Sweet Metroid. It was destroyed very similarly to how a Metroid would be, when an Ice Beam Item Sphere was lobbed at it by Kevin Keene, followed by shots from Kraid's Missile Gun.
The creature's name may be derived from the Metroid's bio in the instruction manual for Metroid:
"This protoplasm in suspended animation was discovered on the planet SR388. It clings onto Samus' body and sucks his energy. It can't be destroyed directly with the normal beam. Freeze it with the ice beam, and then fire 5 missile blasts at it."