Matt Whiting wrote:
The float bowls were covered with a green slime that looked like
algae.

Interesting. A web search suggests it could have been algae, but the web
site below (among others that have similar text) claims what you dealt with
was (probably) a formation of wax and asphalt. I can see how the latter two
would be a pain to clean out! Did you try anything like the product known
as "Goo Gone"?

http://dieselfueldoctor.com/fuel_solutions.html