And these Gadianton robbers, who were among the Lamanites, did infest the land, insomuch that the inhabitants thereof began to hide up their treasures in the earth; and they became slippery, because the Lord had cursed the land, that they could not hold them, nor retain them again.Mormon 1:18
I don’t think that Mormon means that the treasures literally “became slippery”. I think he only means that figuratively.
When something is slippery, it slides out of your hands easily. My guess is the Lord made it so the treasures slid out of their possession easily. How? Well, maybe the Gadianton robbers found the treasures, even though they were buried. Maybe the robbers said “I’ll help you hide it” or “I’ll trade it for this” but didn’t keep their word, and instead kept the treasures. Maybe some actually did “become slippery” and people kept dropping them and never finding them again. But I think that the Lord worked in many different ways, using different methods all the time to make them “become slippery”.