This plant appeared in my tortoise enclosure about 2 weeks ago. At first I thought it was lettuce, but I am not sure. I haven't let my tortoise eat it.
There's a difference between toxic and 'do not feed'
According to the Tortoise Table Plant Database: "All the plants from the Polygonaceae family contain high levels of oxalic acid which binds with calcium and other nutrients preventing their absorpbtion, and possible cause of kidney stones and gout; so best to avoid."
Link: http://m.thetortoisetable.org.uk/m/plants_19.asp?mode=main&catID=131
A few bites won't kill, but it shouldn't be fed. Joe doesn't bother with it which is why I suspect your tort won't either.
However, Dock is a pain to get rid of because it toots so deeply. It will get much bigger and uglier. As it's of no use to your tort, I recommend you dig it up now.