Any idea what to use to clean a rain Barrel?

Hi, bi carb soda is great and you can add white vinegar to it for a stronger mix, it is safe for septic systems and maybe a pressure wash would help

Vinegar and water. Let it sit for a while and then scrub and rinse.

Vinegar, plain old vinegar. That is what my friends use.

Hydrogen peroxide might be a good solution. I have used it to get algae slime out of my bird bath by adding a full bottle to it. It is nontoxic to birds so should be safe for your rain barrel. Partially drain and rinse away all the yuck you can from the barrel with a garden hose then add the peroxide.

Try baking soda and aluminum foil to scrub