The temp is a bit cold for him and this will decrease his appetite. I would raise the temp to entice him...probably in the 80F range. The growth itself could be a couple things. He may have had an injury and it is scarring from that...sometimes it will look like keloid deposits are happening on these guys...just kind of over healing. It could also be a sebacious cyst or even a cancerous growth. I would, as long as theres no change in shape size texture or color, watch it closely and as long as he has no discharge and its not bothering him, then it should be ok. I would however, keep him completely seperated from your other turtles, washing with bleach water everything he comes in contact with.You don't want to risk infecting your guys with anything he has.
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Let me know if you have any other questions
Good Luck &
Best Fishes