Okay, since he eats live my best guess is he took a bite to the face during his last feeding.

You can put your thumb under his chin and gently move it to the left or right. This will pull his lip down a bit and let you better see the extent of the injury. That is not a good color. Assuming the last feeding before this one was about a week ago and you didn't notice immediate swelling and bleeding under the gums, this is probably an infection. IMO this is beyond the scope of what you're going to be able to deal with at home.