The green stuff is the growth particle effect — it means the baby mob is being fed and is growing up faster.

What it means

  • When you feed a baby animal in Minecraft, green particles appear.
  • That visual cue shows the mob’s growth is being accelerated, so it reaches adulthood sooner.
  • Players online also describe it as the same kind of “good” particle effect you see when a mob changes state or is affected positively.

Simple example

If you give a baby cow wheat, the green particles show up and the cow’s maturity timer goes down.

Extra note

Some newer Minecraft guides also mention a separate item that can produce similar green particles when used on baby mobs, but the basic “feed baby animal = green particles = it’s growing” rule is the one you’re asking about.

TL;DR: green particles mean the baby mob is getting older faster after being fed.