He didn't signal play on. Play on is signalled by two arms pointing, he had one up. He saw Haaland flying, didn't expect him to get back on his feet, so he blew the whistle and called a foul. So the defenders paused. Grealish is only free because he kept playing despite the whistle being blown. So he should have let play continue, but it wouldn't have been Grealish free on goal if he hadn't blown it dead.
He didn't signal play on. Play on is signalled by two arms pointing, he had one up. He saw Haaland flying, didn't expect him to get back on his feet, so he blew the whistle and called a foul. So the defenders paused. Grealish is only free because he kept playing despite the whistle being blown. So he should have let play continue, but it wouldn't have been Grealish free on goal if he hadn't blown it dead.