Well, get yourself a good pressure washer to clean the surface. Let it dry for a few days, and I mean a few days, because if you have moss growing on your deck, you aren't getting ANY sunlight on it! No?
You need to buy yourself a good sealer that touts to prevent mold & mildew (and hopefully moss too.) I can't read the cans/containers from here but that's what you want. If you buy a product whose strength is UV protection, well, I thing your wasting your $$$ (UNLESS you trim back some tree branches.)
I don't want to ruin your 'getaway deck' but I think you need to at least improve the sun and air flow 'situation'. Otherwise, you'll be doing this job every year. BUT, that's OK w/me - You'll be a Pro in no time!
I want to commend you on your choice of Mfgrs. They're 2 of the best. Shop at that 'level' and I'm sure you'll find something to satisfy your needs. My best to ya and hope this helps.