I think it's Sedum Acre....It's growing all over my enclosure...It's a ground cover thats very invasive, and high in Oxalic Acid. I've never seen Pio eat it or my boxies either. Does it make little yellow flowers?
Is this it? http://www.thetortoisetable.org.uk/site/plants_19.asp?catID=45
Now that I'm looking at it I don't think so.