Curious, in Spanish we have the same saying, but always in the negative version ("no hay moros en la costa") which is something you say when you're doing something secret and there is no one around who could see, hear or cause trouble.
In the UK we say 'the coast is clear' when telling someone that 'there is no-one around to see any misdeeds you're about to do'. Nothing about Moors, nor even Spaniards!
In an alternate timeline, The Sea Peoples are Romans sailing to England, the Anglo-Saxons, the Normans. Things became fuzzy when the English themselves became other civilization's Sea Peoples.
I wish the author would go into detail about the sea peoples. From what I read, one theory is that they were subject allies of the Hittites; once the Hittites collapsed they went in search of better farmland with their entire families.