what is on my clownfish

[Denise0409]

I am new to the saltwater world. I am cycling my 1st tank. I have 2 small clown fish in the tank and picked them up on Sunday. I understand the cycling process. I have experience with freshwater. Anyways. I picked up the fish on sunday. There is a black mark on my fish by his back fin. Not sure what this is. Anyone know

 

[Gumbies R Us]

Is it the black mark on the tail? having trouble figuring out what I'm looking at