It may be a good idea to get him seen by a vet just to make sure he's ok. It may also ease your mind as well. As knowing what it is exactly and how to take care of it is never a bad thing as it could be anything.
My male kitten got nurtured on Sept 19, he has one incision on each of his testicles. The left testicle has a black circular mark on it now, what could this be? I’m not sure if it’s apart of his incision, or maybe poop stuck? Should I try to wash it off with a damp towel?
The picture is a little blurry so it’s hard to tell what it is. You could try using a warm (not hot) damp towel very gently. If nothing comes of that I would recommend asking your vet.