........and the mould on the inside of the tank is probably alge.
We get it inside our tanks that are clear plastic and the are exposed to the sun. We wash it out with clean tap water using a cloth taped to the end of a broom stick and then jetted out with the hose pipe. You will probably be able to rig something up with a thinner diamt rod to clean your barrells. Someone suggested that they are filled with water and some Milton and left to stand for a few hours then rinse out to clean them.
A strong concentration of water mixed with bleach should also work. Empty water into the drains and keep away from eyes, skin and clothing.
This mould doesn't appear to alter the TDS of the water and doesn't interfere with the clean unless the water begins to get slimmy.
My van now is fairly dark inside with blacked out windows, full steel bulkhead and an old duvet cover over the tank, so I now clean it out about once a year if that. When I had the tanks on a trailer I had to clean them every couple of weeks.
Spruce
Added.
The chaps who use hot water systems heat the water as it is pumped to the windows. Some of the guys were heating the water in their tanks with electric elements, but this won't be the cause of the growth, although it may grow quicker because of the warmth.
The chlorine in the water is what kills alge and we remove the chloine before filtering the water through the RO. But our water is exposed to the air which also carries these alge spoors in the air. As we empty our tanks air is drawn in from the atmosphere together with any particles in the air. No chlorine means that the spoors grow with the aid of sunlight.