Whis is really a consistent impartial character in the Dragon Ball Super anime. He's always respectful and respectful at anyone whatever the person's status. Even though food is just one of those few matters which makes him reveal his real character longer, he can also be seen grinning when he watched a great fight. That is true especially if this fight involves one of his trainees. Goku's Ultra Instinct made him blush like that.
The hottest extended preview of Dragon Ball Super episode 129 shows Whis in a rather different shade of enjoyment. After visiting Goku breaking up his limit once more, Whis was spotted smiling an extremely knowing grin. Perhaps he knew back then he succeeded in turning out a mortal to a god like monster.
This smile is an extremely out of character to get Whis, as he does a menacing grin like that. He just does that when he knows something which no body does. This smile is supposed to happen Throughout Goku's transformation from Ultra Instinct Omen to Mastered Ultra Instinct. Because we've just seen the visual so far, we still don't have any idea in regards to the Mastered Ultra Instinct ability.
Whis' reaction could mean so many matters. It can indicate Goku finally owned Ultra Instinct. In his first transformation, the trigger for this is his absorption of this energy from the Spirit Bomb. The next time he transformed, it had been Kefla's enormous power that awakened the conversion. The third and current transformation is triggered due to of Jiren's overwhelming power.
The Mastered Ultra Instinct will probably be because of Jiren's overwhelming power, even to Ultra Instinct Omen. There's a scene in the preview where Jiren protected himself against UI Goku's Kamehameha. Next, Jiren gave it straight back to Goku, the Saiyan received an immediate hit. Since Mastered Ultra Instinct looks, Whis realized that Goku is now liberally on his level. If he accomplishes it, then he can now be considered a God of Destruction at the building, just like Toppo. Even more, Goku could be more powerful than the U11 GoD offender; and Whis smiles because he understands.