1) The God in question has this Favor Marker/Icon because you still have to unlock it/buy it.
2) The God in question is already unlocked/bought, but it's showing you what Skins are purchasable not only by Gems, but Favor as well. (So that little Favor Marker/Icon shows you what Skins have both options; Gems or Favor.)
BIG EDIT: I believe the Favor Marker indicates if you are able to purchase the Skin regardless of what type of currency is necessary. I was taking a look at my Gods List, and it turns out that my God Rank 1 Isis has the Favor Icon on her Gold Skin, but my God Rankless (0) Tyr has no Icon on his Gold Skin. The only thing I can conclude is that the Icon is shown for any Skins you are able to purchase regardless of the currency!
Just to be safe, I checked this against various other Gods that are Rank 1, and Rank 0 respectively and it turned out to be the same, so I'm speculating as much - thus the conclusion above.