You could try shock treating the fuel with a product from Biobor called "Diesel Doctor" and see if that kills the algea. You will need to replace the filters, and likely the fuel tank strainer afterwards. If the infestation is profound, a shop should drain and clean the tank. But try the Diesel Doctor first. Diesel Doctor is probably available from PartsShop, and comes in something resembling a toothpaste tube. Caution, even inside the sealed tube, the product smells like a graveyard of rotting bodies. :0 It gets even worse after you twist open the cap. I had a modest algea problem with my car, and it solved the problem. Good luck, and don't forget to put a clothspin on your nose.
Regards - Luke
Saint Helena, CA