[cypher]

That looks like a bruise or bleeding under the skin, could he have trapped it somewhere? Any toys with small gaps or chains he could have caught it in?
Is he in a barred cage or a tank, if it's barred he might have caught it climbing.
If it doesn't seem to be bothering him he may not be in much pain with it & if it is a bruise then it should heal with time but I would give the cage a good check over for any possible places he could get his feet or nails trapped.