@Lelouch, this is a very interesting observation.
Messana, the female figure driving the mule-biga, is described by most of the numismatic scholars as a civic personification of the city (e.g. Imhoof-Blumer, Nymphen und Chariten auf griechischen Münzen), or, by others, as a city-goddess (e.g. "Stadtgöttin" in Roscher’s Ausführliches Lexikon der griechischen und römischen Mythologie).
To my knowledge, there is no clear literary or cultic evidence that Messana was regarded as a nymph in the strict sense. However, her association within the nymphic iconographic sphere — notably Pan, a water source, and the nymph Pelorias — may explain why some authors identify her as such.
In fact, Messana is explicitly called a nymph by modern numismatists, not only by Hoover, but also by Westermark (the coinage of Akragas, p. 137), who writes: “At Messana the legend MESSANA designates the nymph driving her biga.”
This usage seems to reflect an iconographic and functional interpretation, rather than a securely attested mythological or cultic tradition.