After mistrials to wash off the mildew I decided to attack the problem by sanding it off. This works well but is a dirty job.
Luckily there is no water in the engines oil and it turns over but it won’t start. After draining the tank I end up with 15 gallons of water and 5 gallons of sour smelling diesel fuel. Now I’m just hoping there is no water in the engine’s fuel pump.