Who would be a good flex option running back for this week?

I would say it's between Williams and Green-Ellis. Green-Ellis won't have to split carries, but Miami's defense has been strong. Snelling is a back-up until Turner gets injured, so don't play him yet, and Royal has a tough match-up this week, but then again you never know with fast wideouts. As long as you don't play Snelling, you can't go too wrong.