It is a bladder stone, and you are lucky he passed it. This is your wake up call. The next one could kill him if you don't change up your husbandry.
These "stones" are a by-product of protein digestion caused by dehydration. They are not from foods containing calcium or oxalates. Protein is necessary for tortoises in the correct amounts, so don't think you need to cut out all protein. Instead you need to increase hydration. More soaks, spray water on the food, humidify heated night shelters or indoor housing in winter when the heat is running more, and if the tortoise is indoors, humidify the indoor enclosure more.
