This is Hedorah, the Smog Monster! From 1971's Godzilla vs Hedorah.

It changes the slime to Hedorah. The Good thing about this is that slimes have modifiable size, so you can make, the initial, microscopic form of Hedorah, as well its giant, perfect form!

It will also be added to my datapack as a new mob put into the game.
Click here to see it: www.planetminecraft.com/data-pack/cdd-crewmate-s-creatures-datapack/

(A higher definition version of this can be installed here. It's also the version seen in the datapack.)