Figured someone on here will be an expert at this. We have light brown trex for deck boards and part of the deck is covered by roof overhang. Where it isn’t covered snow sat all winter and slowly melted this spring and now it is stained some with this mottled black. Not sure what to call it, mildew, mold whatever. I got a stiff brush and tried to clean a patch with some mild soap and it did nothing.

Should I pre-wet it? Wait until it’s dry for a few days? Any special products out there?

I would suggest using a pressure washer with mild soap.

1 part bleach to 4 parts water, shot of Dawn dish soap. Spray on and let sit for 10-15 minutes. Scrub and rinse or GENTLY pressure wash. It should clean right up. If it won’t come completely clean the first go around, reapply the cleaner and let dwell a bit longer.
We clean lots of composite decks.

^^^^^^this^^^^^
Or TSP and some elbow grease. If using a pressure washer? Don’t use it full blast. Being in a shaded area. You are going to have to do this @ least once a year.

If it's mold, get some wet n forget

It is mold. Trex has a history of this, and if you contact the company, they will acknowledge that. You can apply any of the mold inhibitors out there, and you might have some success, however it will be temporary.

Power washer. Comes right off.

X2

It is mold. Use Olympic Deck Cleaner. I do mine once a year with it.