I've been using a set of D'addario XL's on my bass for about a year and a half ( that is, since I've been playing! ) and I've noticed there's some fungus on the back side ( the one that's faced upon the scale ) of the low strings ( B and E ). Those strings are expansive, and can last for about 5 years - so I was told! Well, there's not any sound disturbances... yet. So I dunno... is that natural or not? If it is, how can it be softened?