It looks like pigment is changing which is normal. Some black spots could come from coral. Could come from stings from Zoas or any LPS. But nothing to worry about.
Hyper Melanization is the word that is used when clowns get this.
I noticed the same thing on my clown today. From what I have read it may have been stung by a zoa or something else in tank. Every thing I read says it is nothing to worry about.
The clownfish looks as if he has been rubbing against a coral. Those are usually sting markings.
Do you have any LPS coral that he may be possibly trying to host?