28th-minute strike proved the difference as we dominated possession against the Bluebirds without carving out too many clear-cut chances.
Steven Caulker struck the woodwork for the visitors but Hugo Lloris was rarely tested – Michael Dawson led the defensive line brilliantly to keep Cardiff’s late aerial approach at bay.
“We didn’t play that well but the most important thing is that we won the game and we stay positive,” said Mousa, who has now joined up with the Belgium squad alongside team-mates Jan Vertonghen and Nacer Chadli for a friendly against Ivory Coast on Wednesday.
“We could have won by more but Cardiff are a good team and they fought hard. They made it difficult for us.
“We’ve had a lot of games lately and if you can win matches when you aren’t playing well, that’s a good sign. We were all determined to win this game.”