http://www.possumpages.com.au/newgipps/turnip.jpg That's a turnip. Iguanas love the greens usually. Make sure they are cleaned well. They do have small spines on the leaves but most turnip greens in the grocery store will already be cleaned.
I've never heard of anyone feeding the leaves before. If it's as closely related to cabbage as I think it is, then I wouldn't offer it at all. I'd stick with the normal greens: turnip, collard, mustard, etc. Just to be safe.