Technically speaking, I don't advocate the use of chemicals if necessary, but having such items on hand can be a life saver. Since the "growths" are disappearing, there is little worry because I think what happened was it just gathered some dust particles on its slime coat.
I don't know if the acidity caused by the decomposition of the leaves will retard the growth of good bacteria, but it does reduce bacterial activity over time. Since tanks that have pH around 5 are still able to be fully cycled, I believe the filter bacteria should be resistant enough to acidic waters, as long as it doesn't plunge below 4.5 or so. It is likely that whatever bacteria that survives are typically acid-resistant bacteria, which should not harm the fish. Long term, the presence of acidity and other compounds released and caused by the rotting leaves will aid in the recovery of sick fish like a Betta.
In a small container, you only need a small piece of ketapang leaf no bigger than a 50 cent coin. I find that using any bigger, the water will turn super brown, and pH in such containers may be below 5.
You should attempt to feed the Betta to see if it responds to food. Just a little bit to test his appetite. If he doesn't eat the food, remove it immediately.
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