  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.
