I don't know if this is official, but it appears in Hesmer's book. France's traditional national cross - equivalent to England's red cross on white. It was a French flag and was surely abolished when the British took over those colonies. The unofficial Martinique flag is not especially associated with aspiration to independence.
Independentist movements use totally different flags.
Anyway, the snake flag, often improperly considered as the departmental flag of Martinique, was once official. The snake flag is considered locally as a 'blazon', and is not available locally as a flag. It is mainly erroneously used as a courtesy ensign by yachtmen, which should hoist the Tricolore civil ensign. The snake flag was adopted as an ensign in 1766, and was nothing but a variant of the civil ensign used by French vessels. Such ensigns were also used in the French ports.