I'll bet it was "just a wing panel coming loose". Although it should not happen, it's a long way off from a cracked wing! If a wing would crack to such an extent that the pax could see it, he [or she] would not live to tell.
If it's only ONE wing panel, its perfectely safe. Airliners are designed and proven safe to fly in this condition. But I admit, if I would witness such an event [having a degree in aeronautical engineering], I don't think I would be 100% comfortable...
Immigration officer: "What's the purpose of your visit to the USA?" Spotter: "Shooting airliners with my Canon!"