a power washer would clean it off and remove debris that has accumlated. BUT, if its cedar, the wood is too soft and the washer would do more damage than good.
I would consider using the product from the Flood Company that is a great deck wash. you can spray it on with a deck sprayer and let it sit, then wash it off with the garden hose. This will work well.
Then after that consider using another product from the Flood Company called CWF-UV. you can get this in clear or cedar color. It is NOT topical like most sealers and will soak into the wood and give several years of good service. This product has to be painted on, you can spray it on and back brush, but I find it easier to just brush or roller it on.
You will be very happy with the end results.
First, I would find out if the deck is cedar for sure.