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The only sure way to rid the problem is to get the contents of the fuel tank “polished”.
Most marinas that service large diesel boats can do that.
The final solution is to prevent algae. If possible keep the fuel tank filled, treat with an algacide and if possible keep the bus inside where temperature variations are minimal. Obviously change fuel filters frequently.
Any treatment that kills algae is likely to dislodge any that might be on the tank walls so you may wish to delay polishing but change filters often initially. If you keep moisture out of the tank you will not grow algae.