there is just no single straight forward answer to this question, you can apply chemicals, you can sand, you can just pressure hose it with water, it all depends on your actual coating and the wood which is underneath.
If you use some chemical , then this may create stains and then when your coating is gone, your decking has now a problem with stains, which are either superficial or profound. Then you can use bleach, but this may degrade your valuable wood. Anyhow, what to do, depends on you, but if your deck is expensive then we recommend a professional deck renovating company. It is really not that simple.
As a hardwood exporter (both decking and flooring) we do receive questions in respect of installing and finishing and for the future maintenance and cleaning, here we have listed the different kind of stains on your deckingand advise on how to remove them and further advise on cleaning in particular with respect to a product called sodium percarbonate which is good for removing from your deck the mold, dirt and dead grayed-out grains