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!