Jet-A, Jet- A1, kerosene, diesel contain a small amount of water.
These fuels are hygroscopic, meaning that they have the ability to attract water in the form of moisture from the atmosphere.
Since water is heavier than these fuels, it sinks to the bottom of the tank.
The algae you see in the fuel is usually a sticky black slime that grows in the water at the separation level of the water and the fuel above.
Algaecides such as Biobor kill the algae present in the fuel stopping it's growth.
Should you see algae in the fuel, simply pump and filter off the good fuel above the algae and dispose of the rest. There will be very little water and fuel containing algae remaining. Storing fuel at a constant temperature around or above 70° F in closed containers will prevent condensation from forming in the air space above the fuel.