I suspect, as others have said, that this is a hybrid. But it's at least possible that it's a mutation.
You'd have to let it grow out, and possibly set fruit, before reaching any firmer conclusions, and that could take a very long time, yes? On the other hand, if it sets fruit relatively quickly, it's almost certainly not pure yuzu, which can take up to a decade to produce.
You might have a very valuable plant there, in terms of interest and possible future breeding rather than money, regardless of whatever it turns out to be descended from.