Like joesmum said it is most likely a battle scar, it doesn't really look like fungus or anything threatening just an old wound that healed up kinda funky. My redfoot has a battle scar too, it doesn't bother him at all.
Thanks
Tyrell is indeed a skid row tort. He literally rolled up off the mean streets of LA about 6 years ago. Tough guy that little dude, battle scars make perfect sense. His first home here was a small enclosure made of tombstones, good to know he will not be needing one of his own.