Before taking the tank or pickup out, get some Algicide killer for diesel fuel from most any decent auto parts stores. They used to not carry the algiced in small bottles but they do now. Dump it in your fuel tank and drive it a little while to treat the whole fuel system.
Not winter anti gel, not that at all.
They make a product to kill this growth and use it really strong at first treatment to KILL and then you use it in lower amounts for maintance.

That way if it's a problem, you already did the first part, KILL it! Then you have to clean out all that dead bacteria or whatever it is.

Otherwise it's very contagious and will spread from the top inside of your fuel tank if it's not all treated at once, it'll regrow without treatment.

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