Flag of Adal Sultanate.
Own work, based on the description of Castanhoso:

"...the king of Zeila [Imam Ahmad] ascended a hill with several horse and some foot to examine us: he halted on the top with three hundred horse and three large banners, two white with red moons, and one red with a white moon, which always accompanied him, and [by] which he was recognized." (Extracted from: The Portuguese expedition to Abyssinia in 1541-1543, page 41  at [1]).
FDRMRZUSA