The goblin fails its save and falls prone! In 5e, being prone means the following:
First, it can only move by crawling unless it stands up, which costs twice as much movement. Within the difficult terrain of the
grease spell, that makes quadruple. Second, it has disadvantage on all attack rolls because of its poor positioning and mobility. Third, attack rolls against it have advantage if the attacker is within 5 feet, as it's harder to move out of the way, but disadvantage if attacking from farther away (including reach weapons), as it presents a smaller target.
You can stand up from being prone by using up 15 feet of movement, but only on your turn - so ideally, the goblin won't get the chance.
Here's an updated map:
It is worth noting that Zuzie can't close into melee with Pieron and the goblin is a single move, so she would have to dash (using her action) or make a ranged attack at disadvantage (negating her sneak attack damage) if she wants to attack the goblin right now. It's possible to delay your action if you want to see if Pieron flattens the goblin with his advantage before deciding what to do.