Cang (苍) means grayish white and in this case it's last character of ML's name Dongfang Qing Cang (东方青苍).  Cang (苍) is also part of place name 苍盐海 where the Moon Clan is located.

Lan (兰 ) means Orchid, which refers to FL (name and the flower type she grew from).

Jue (诀 ) means either trick of the trade or parting / farewell.

Cang Lan Jue (苍兰诀 ) is the name of the original novel and also the native name of this drama.  The name is likely refers to the separation between the ML and FL and their love and responsibility they faced.