This may be a cyst. I breed red ear sliders and a few of mine have come down with this same thing. What I did was take a sterile and very sharp razor blade and made a small cut in their cheek. The swollen cheek was caused by a corn kernel-like cyst. I removed it and after a few days the turtles were healed and perfectly fine. This was over 10 years ago and they are still alive and having babies. Good luck!