I'm starting to think that was a defect in some of the hosing. I have 2 50 footers both around the same age. One is almost completely black the other looks new still. That's my guess
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Harmless to fish, but if it bothers you then you can probably dry it out with an air compressor or run some diluted bleach solution through it on a regular basis.
Mine gets it too. I run hot water through it every now and then when I'm finished with water changes, it doesn't really clean it out but it makes me feel better. I had it rolled up in the garage when it started so I kept it outside during the summer. No change though, I just figured its some harmless algae.
This is not mold - but algae. It's very common - and harmless. Over time, as it cakes, flakes of it will just fall off on itself, but not much you can really do.
If it really bothers you, you could use a tube cleaner (piece of flexible springy metal with a brush on the end, and spend hours scrubbing - but it will just come back.
Just live with it - or keep buying new hoses.