Golden Care Teak Cleaner uses Oxalic Acid, a mild cleaner. Another product from Home Depot called Behr Wood Cleaner is essentially the same thing; though a larger jug is available, at less cost. Or, you could buy a bag of oxalic acid powder on Amazon and mix a bucket full for pennies on the dollar. Either way, oxalic acid works great on Teak. Apply, wait, rinse. Then rinse more. Then, a little more to avoid a nasty residue difficult to sand off, wastes money, and time.
Gloves, mask, and goggles. Agitating helps. I’d let it sit for a day in the sun before rinsing. Tends to give a more blonde, less honey look, so I only use it when necessary to eradicate stubborn growth or mold.
PRO-TIP: Leave in the sun for a couple days after.