It could be harmless but there's a chance that it could be wool-sucking. This is a compulsive disorder in cats which is quite common, but really it's not too hard to manage. However, if the cat is allowed to wool suck too often, it can ingest the material which can become impacted in the stomach or intestine, and this requires surgery to remove. If you just keep an eye on your cat when he's doing this though, I'm sure it'll be no problem.