that looks like the sickness they always suffer is it like a pimple or excess of tissue if its that try to get medicine for him or he will eventually die
usually it wont u can try a copper based treatment or quick cure but u should ask a fish pro cause i dont really remember what it is called but i know the clowns suffer from that since i have had that trouble in the past and the passed out
3 of them. One was the result of my serpent star. The other two were from another LFS that I don't buy fish from anymore. This place I buy from now is a good place.
Clowns I think are sensitive to copper.......I am not sure but why don't you try to catch it and take it back to the store that sold it to you.....???? just a thought