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.![]()
I know of people that have had pythons for years that are squeaky clean. I have 2 the same age pretty much. Both get used regularly. One is caked with it one isn't. That's why I suspect the hose in general. Maybe some batches are more porus then others perhaps
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