Azure damselfish has black anus

[luke101emp]

Hi

 

I have 3 azure damselfish in my tank one has got a black anus sticking out. I first thought it was poo just stuck to him. It has now been there for a week, he has stopped eating and has a swollen belly. He is swimming fine and is still defending his rock. There are no other markings on him or any other fish. I have read about worms but they say they are white or red. I have attached a pic of him. Any help would be great.

 

My tank readings are all great nothing has changed for months so it has nothing to do with water or anything like that. Also my other fish are all fine 2 clowns 1 fox face 1 green mandarin 1 ghost goby. No markings on any fish no funny poo from any fish to say its worms.