Yogurt has mold

I bought a Parmalat yogurt at Spar and left it in the fridge. The next day, my son called me to ask if he could eat the yogurt, and I said yes. Later, he called again and said the yogurt didn’t taste right. I was surprised, so I asked him to take a picture of the expiry date. It showed 25 February 2026, so I told him there shouldn’t be anything wrong with it and that it should be fine to eat. When I got home and checked the container myself, I saw that he couldn’t finish it. To my surprise, the yogurt had mold in it. How can this happen before the expiry date, and should we be concerned about our health in this case seriously?