I was told that you just spray water/bleach on a couple times and over a week or two, let it sit and it kills the green algae. You then wash/sand it to a nicer finish. I’ve always just pressure washed decks using a 2700 psi pressure washer. It was pretty fast.

And for sealing it I would not recommend Thompson’s clear water sealer. There are two kinds, one that allows the wood to turn gray and one that doesn’t. I got the one that was supposed to keep the wood looking new but my oak still turned gray.