As a matter of practicality, you will spill/spatter grease again in the future. Chemically removing grease from the wood removes some of natural wood oils, damaging the wood and making the surface more vulnerable to sun and mildew. Probably best to clean once (standard deck prep) and refinish with an oil based stain/sealer. Standard cleaning procedure should get some of bad grease out. Dark natural color will hide the rest. Shows up much more on grey deck since grey deck is generally dried out surface. Sanding is a lot of work, and if wood has aged enough, grease may penetrate quite deep.

You could try wiping it down with a little acetone. That should remove just about anything hydrophobic (=oily), and my paint guy recommends it for de-oiling ipe decks prior to staining, so it's apparently safe with wood.

When I hear of someone using cedar for railing, I feel I must tell them of my experience.18 years ago the popular belief( at least at my lumberyard) was that cedar was low maintainence and lasts forever. So I sided, trimmed, railed and decked w/ cedar( white cedarsiding, red cedar everywhere else). It was beautiful afterward. We stained w/cabots, used sikins on the deck. After 5 years we noticed, especially the deck and rail, that everywhere cedar touched cedar( the butt joint at the post/ rail and post /deck) it was getting soft. We hadn't sealed the end grain before installation nor did we caulk each joint. By year 12 the railings fell off and the posts and decking boards rotted along each edge ( we didn't space the deck boards enough, thinking they would shrink- they might have even expanded). A word to the wise, wait until a stretch of dry weather and silicone caulk those joints. Fine Homebuilding Magazine recognized this problem and published an article a few years after we built and after the damage was done. They recommended geotextiles layered between built up trim and cutting chamfer grouves at the butt joints for the caulk to adhere better- esentially a miniture pitch pocket for the caulk. Good luck w/ the cedar.

There is no problem with leaving it uncoated. If you give it some more time, the whole thing will eventually grey. If you do decide to restore it, try Restore A Deck and Woodzotic. Those products work really well.