Might be out of luck there. I only saw the one in our care eat fishy, seafoody things and some googling says they are almost strictly carnivorous, and many seem to suspect what little vegetation ends up in their belly is probably just coincidental.

Diamondback terrapins are almost strictly carnivorous but sometimes ingest small amounts of plant material. They possess strong jaws and crushing plates in their mouths that enable them to eat hard-shelled prey, including aquatic snails, crabs and small bivalves, such as mussels and clams. They also eat carrion, fish, worms and insects. (National Aquarium)

As carnivores most Diamondback Terrapins will readily accept any number of commercial turtle diets. We suggest a mixture of three or four different high-quality types. Fish, frozen crayfish or shrimp, and earthworms should add to the variety. Freeze-dried krill or brine shrimp can also provide a great treat for these turtles. (The Turtle Source)