I've never heard anything about carex evergold or Festuca Elijahs Blue being poisonous to animals or humans, but I don't know diddly about box turtles.

Here's a link for a listing of plants NOT recommended as pond plants due to danger to koi and pond life.

Here is a link that might be useful: Toxic Pond Plants

Fescue (and its close relative, Lolium) usually have endophytic fungus, which is sometimes toxic. Some New Zealand grass breeders also breed the fungus to yield better forage for livestock.

Here is a link that might be useful: Endophytes