My fuel tank has diesel bug! It presents as thick slime and looks like an gunky black, yellow green (no not a novelty one) used condom stuck onto the end of the fuel feed pipe, where it sits in the bottom of the tank. It will block the flow and does starve the engine of fuel.

I am currently about two weeks out from wintering the boat in Sicily and am treating it with anti bacterial addatives.
My question is, what should I do with it over winter? How should I treat the tank?
Dave