See if you have a reptile rehabber in your area. Looks like there might be some bone exposed. This turtle likely needs medical attention. A rehabber would know what to do.
I like to stick to feeder insects (live or freeze dried) for protein for my box turtles. Hardboiled eggs are eagerly accepted but only given occasionally. Tuna goes over well, but rarely served. Dog / cat food is served rarely. My main issue with it (and cow meat) is that it makes a boxie‘s...