What was that ? UFO ?

While editing a holiday video from Astypalea - Greece, I just happened to find that I shot something unusual. It looks like an UFO and it's just a moment (maybe a second) it was brutally fast, just so it suddenly appeared over the beach where we were alone and quickly flew away and it looked like a cloud, I have no idea what it was when we were there we didn't notice anything ... What do you think? What was that ?