  We derive direct measurement gas-phase metallicities of $7.4 < 12 + \log(O/H)
< 8.4$ for 14 low-mass Emission Line Galaxies (ELGs) at $0.3 < z < 0.8$
identified in the Faint Infrared Grism Survey (FIGS). We use deep slitless G102
grism spectroscopy of the Hubble Ultra Deep Field (HUDF), dispersing light from
all objects in the field at wavelengths between 0.85 and 1.15 microns. We run
an automatic search routine on these spectra to robustly identify 71 emission
line sources, using archival data from VLT/MUSE to measure additional lines and
confirm redshifts. We identify 14 objects with $0.3 < z < 0.8$ with measurable
O[III]$\lambda$4363 \AA\ emission lines in matching VLT/MUSE spectra. For these
galaxies, we derive direct electron-temperature gas-phase metallicities with a
range of $7.4 < 12 + \log(O/H) < 8.4$. With matching stellar masses in the
range of $10^{7.9} M_{\odot} < M_{\star} < 10^{10.4} M_{\odot}$, we construct a
mass-metallicity (MZ) relation and find that the relation is offset to lower
metallicities compared to metallicities derived from alternative methods
(e.g.,$R_{23}$, O3N2, N2O2) and continuum selected samples. Using star
formation rates (SFR) derived from the $H\alpha$ emission line, we calculate
our galaxies' position on the Fundamental Metallicity Relation (FMR), where we
also find an offset toward lower metallicities. This demonstrates that this
emission-line-selected sample probes objects of low stellar masses but even
lower metallicities than many comparable surveys. We detect a trend suggesting
galaxies with higher Specific Star Formation (SSFR) are more likely to have
lower metallicity. This could be due to cold accretion of metal-poor gas that
drives star formation, or could be because outflows of metal-rich stellar winds
and SNe ejecta are more common in galaxies with higher SSFR.

Astrophysics of Galaxies