Open wound on the head of a terrapin

I have 2 terrapins, and one of them has an injury on its head. It pulled a piece of skin off its head. This skin shred was at first still attached. When I looked for it the next time, it was gone, and it bleeded a little. However, the terrapin was agile as ever, was hungry and reacted when I called it. Then a little skin grew over the wound, but today I noticed that it came off again. I took the terrapin out of the water and put it into the bathtub. I also put some wound balm on the wound. Should I take the terrapin to a veterinarian?  I do not want the terrapin to suffer from pain when I put it back into the water!