Solar Power?
Why else would POTUS go straight to Camp David right after receiving this letter, other than it naming names?
And yes, it does appear to resemble a mason's apron; messing with shadows and highlights clearly shows the edges on all sides being highlighted in a manner similar to the apron.
He -received- the message from Kim.. Maybe the message wasn't meant for us, not every message is, or was shown as a warning to (them); or maybe the message is that Q is in front of/above these masons. The symbolism is there regardless, and yes, it will be their downfall.