Assuming you have anemone(s), an anemone sting will cause this.
On clowns and tangs, often black spot may be embedded flatworms in the skin and fins of affected fish. Although the worms are rather easy to get rid once they appear as black dots on fish, it may be a battle.
Don't panic. You can give this clown a freshwater bath and it may go away. If the dot becomes more than one, you can give it a formalin bath once a day for 4-7 days. I use general cure which contains formalin.