So a couple weeks ago I had the Zebstrika event (I ran from Cresselia to explore and found a shiny Zebstrika on the new Mirage island). I saw this question so I beat the League and went to the island again when it showed up. I have no problem encountering it, just wondering if it can it be shiny since I ran away from it the first time.
Yes, it can still be shiny. The only time any Pokémon game saves the stats (and shininess) of a wild Pokémon across multiple encounters is for roamers, which ORAS doesn’t have. Every time you encounter Cresselia should be a completely unique encounter with an independent shiny value.