Powerwash it let dry and apply paint/stain with roller\pad and long stick
You might want to check my power washing post. I have before and after pictures of our deck after being power washed. https://karupp-did.net/power-washer/
Also, you might want to check out Sherwin Williams deck products. I am always happy with their paints and stains.
Power wash it, you can rent one. We'll be power washing and resealing out fence and deck next spring! (Wish I would have done it this year, but we didn't have the time)
I would power wash it first and then you could paint it with outdoor deck paint or stain!