Well, I guess that datchison already beat me to it.
It's not harmful at all, though, being only unsightly. In fact, it's actually quite beneficial. Too bad it doesn't look good. The saltwater guys have developed the ecosystem filter method for growing mass quantities of algae in a separate chamber out of sight in order to get the benefits without the downside, but this hasn't yet caught on in the freshwater arena.
For now, your best bet is to identify the cause and eliminate it.
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