Both the English and the American Mistletoe are poisonous and contain TOXIC proteins. It is also thought that some species of this hemi-parasitic shrub absorb toxic properties such as glycosides and saponins from the host plant. Because they tend to grow on host trees, it is unlikely that your tortoise will encounter Mistletoe in its natural habitat, but berries (which are particularly toxic) and leaves from mistletoe in oak, apple, and other trees could fall onto the ground and should be removed from the tortoise's enclosure.
English Mistletoe (Viscum album) Santalaceae Family
American Mistletoe (Phoradendron leucarpum) Viscaceae Family