My cat recently developed a black crust around one of his nostrils. It's only the right side, with the left side looking fine, and you can't notice it unless you're just a few inches from his face. There isn't a ton of crust; just a little black ring right on the edge where the nostril meets his face. Does anyone know what this could be? He's always had a bit of crustiness, but this hasn't gone away over the past few days and I just want to make sure it's normal/not bad.