Probably not.
It's impossible to say just from the fact that the app was open what opened it, and even less so when you know nothing about the circumstances. But the most likely was that it was opened by accident when you were putting the phone down, or perhaps while in your pocket (possible if you don't use a security lockscreen, or have some event or trigger that can unlock it such as a "trusted device" or "trusted location"). Even possibly when picking the phone (I always unlock my phone with a fingerprint as I pull it from my pocket, so it's unlocked before I ever see it. This means that if I were using an open-fronted case it would also be possible to accidentally launch an app while picking the phone up). If this is a one-off it is far, far more likely to be something like that than someone hacking your phone and opening the Walmart app (if I were hacking someone's phone I'd be extracting information without alerting them to the fact that I was there, not alerting them to my presence by opening apps on their screen).