Based on the pictures you've posted, this can be a sign of an organ prolapse, immediate veterinary medical attention is highly required. We suggest this, to avoid further complications as well as possible infections.
For the time being, please separate the affected terrapin from that of the remaining terrapins that you have. Provide good quality food and minimizing or limiting their access to terrarium water is important to hasten healing process.
Eliezer M. Bauzon, DVM RN