I agree that it looks like biting. Sometimes they'll stop, sometimes they continue to always chew their tails. :( The stresscoat helps, and feeding high protein food also helps (so that his tail grows out quicker).
I've got two guys who do this. One bites his down to the point that he looks like a plaket...but just his tail. He's a happy, healthy guy...but he eats his tail. :P The other is a double tail, and he only does it once his tail gets to a certain length and it really starts pulling on him when he tries to swim. I really think he's "trimming" it to make his life easier. Annoys me, but keeps him happy...so whatever. :)
He's a beautiful fish though!! I love the variety in his colors, and his little speckly face...too cute! :)