Thanos didn't use the power stone on the Hulk.
Thanos is a massively powerful being on his own, even without the stones. Note that the Power Stone wasn't glowing at the time and also Ebony Maw let Hulk and Thanos fight without any interference, saying:
No, let him have his fun...
To quote an official source:
Infinity War Writers Say Thanos Could've Beaten Hulk, Power Stone or Not, on Movieweb:
Avengers: Infinity War has brought up more than a few debates, but
screenwriters Christopher Markus and Stephen McFeely have now revealed
that Thanos would've beat the Hulk even if he didn't have the Infinity
Gauntlet with the Power Stone, fueling more debate. The opening scene
takes place on the Asgardian refugee ship after Thanos and his Black
Order have taken it over to locate the Tesseract. In a last ditch
effort to defeat Thanos, Loki sends the Hulk into a rage to fight.
However, the fight doesn't go in the Hulk's favor.
Stephen McFeely believes that the Mad Titan would've destroyed the
Hulk with his own bare hands in Infinity War. This would explain why
Ebony Maw allows the fight to continue and makes the scene look like
Thanos was just having some fun, like a cat playing with a mouse.
Looking back at the scene, Maw and Thanos are pretty calm, even when
it appears that the Hulk has the upper hand in the battle.
McFeely had this to say when asked if Thanos could still beat Hulk
I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).
The article further explains:
However, the opening Infinity War scene wasn't just to show that
Thanos could beat the Hulk. Stephen McFeely and Christopher Markus
wanted to establish the fact that Thanos is the biggest villain in the
entire MCU. Marvel Studios boss Kevin Feige revealed early on that the
first 5 minutes of Infinity War would show off the immense power of
the Mad Titan and he was not kidding around.
McFeely explains, We want to announce Thanos as the biggest villain in the MCU, so he
takes out the previous reigning champion. And by defeating Hulk
relatively easily and savagely, hopefully there's a sense of dread
over the course of the rest of the movie for anyone else who comes up
You can read more in the interview with Christopher Markus and Stephen McFeely here.