content: I'm thinking ice.    I don't know what I'm talking about but... I believe the diffuser is chilled, you can see what looks like radiator piping going into it and can almost see individual tubing inside the diffuser itself.  I think it works like a condenser in your AC. Large enough engines like this normally have water sprayed at their opening to create a kind of shield or pillow to soften the concussive sound waves, to help protect the tower/pad/test rig from damage.  The water is normally misted in a few seconds before launch.    If this is a test firing, they might have been spraying the water repeatedly for a while, to do multiple on/off test firings.  That would allow some water to land on/in/around the diffuser before and after firings.  If the diffuser is chilled enough, that water could reach freezing temperatures and icicle down the edge of the diffuser. I'm willing to bet the bottom edge of that diffuser doesn't get terribly hot mid-firing because the force coming out of it pushes the heat away.  As you can see there isn't a lot of light radiation coming from the flame, at least not in the visual spectrum.  I'd wager the most radiative part would be further down the flame.  That would allow the ice to not immediately be melted before the photo was taken.  (Note, that isn't to say that the inside of the diffuser isn't incredibly hot.) Also decompressing hydrogen and oxygen is extremely cold, any piping that contains liquid/gas that is decompressing would become VERY cold and ice over if its not insulated properly. But again, I don't have any clue what I'm talking about.