Context. Known Mars trojans could be primordial small bodies that have remained in their present-day orbits for the age of the Solar System. Their orbital distribution is strongly asymmetric; there are over a dozen objects at the L5 point and just one at L4, (121514) 1999 UJ7. Most L5 trojans appear to form a collision-induced asteroid cluster, known as the Eureka family. Asteroid 2023 FW14 was recently discovered and it has a robust orbit determination that may be consistent with a Mars trojan status.
Aims: Our aim is determine the nature and dynamical properties of 2023 FW14.
Methods: We carried out an observational study of 2023 FW14 to derive its spectral class using the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias. We investigated its possible trojan resonance with Mars using direct N-body simulations.
Results: The reflectance spectrum of 2023 FW14 is not compatible with the olivine-rich composition of the Eureka family; it also does not resemble the composition of the Moon, although (101429) 1998 VF31 does. The Eureka family and 101429 are at the L5 point. The spectrum of 2023 FW14 is also different from two out of the three spectra in the literature of the other known L4 trojan, 121514, which are of C-type. The visible spectrum of 2023 FW14 is consistent with that of an X-type asteroid, as is the third spectrum of 121514. Our calculations confirm that 2023 FW14 is the second known L4 Mars trojan although it is unlikely to be primordial; it may remain in its present-day "tadpole" path for several million years before transferring to a Mars-crossing orbit. It might be a fragment of 121514, but a capture scenario seems more likely.
Conclusions: The discovery of 2023 FW14 suggests that regular Mars-crossing asteroids can be captured as temporary Mars trojans.

Based on observations made with the Gran Telescopio Canarias (GTC) telescope, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (program ID GTC31-23A).