Spinach can be fed occasionally but it contains calcium oxalates which inhibits the absorption of calcium. If you have some and dont feed it alot then it should be fine to feed it, but not often.
Interesting, my tortoise sheldon eats everything i put in front of him. i swear, if the plate was green he would eat it too!! i feed him spring mix and try to pick out as much spinach leaves as i can. if i take him outside for a walk it consists of stopping every few inches to eat another weed or grass. not much of a walker i guess