Water Tank Gunk/Mildew on Sails

[Marcvet]

I have SS water tanks that have gasketed aluminum inspection ports. When the ports are removed the inside of the cover and the water tank rim are covered in a clear almost jelly like slime that is mixed with a hard grey/white material that appears like small gravel. I figure that the grey stuff is some sort of aluminum corrosion but can't figure out what the clear slime is. I never put chlorine in the tanks. Any ideas???

Also what are people using to clean black mildew off sails???

Thanks for your ideas,
Marcvet
s/v Gantman