Fuel tank algae
You should be able to remove the fuel pickup to clean that. You might want to consider cutting a hole in the top of the tank and cleaning it the best you can from there and installing and inspection port in the hole when you finish. You will also need to address the cause of the algae. This is usually from water entering the tank from a leak, a poorly placed tank vent or condensation. The algae can''t grow without it. There are also additves that prevent the algae from forming and you should use them. Biobor is one that comes to mind. These additve claim to break up sludge as well, but I can''t confirm they will actually do that. I don''t reccomend it but I do have a friend with a similar problem whose short sighted solution was to shorten the pickup tube thus keeping the sludge on the bottom from being sucked up.