We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at $sim 65$~kpc from the S0 galaxy IC~550 at $z=0.0165$ ($Dapprox 74$~Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ($M_{B_{max}}approx -17.9$ mag), fast-declining Type Ia with color stretch $s_{BV} approx 0.6$ and $B$-band decline rate $Delta m_{15}(B) approx 1.77$ mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow H$alpha$ line in emission that is resolved in the first spectrum with $rm FWHM approx 1200$ km s$^{-1}$ and $L_{Halpha} approx 3.8times 10^{37}$ erg s$^{-1}$. The detection of a resolved H$alpha$ line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the non-degenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the H$alpha$ line would be $sim 10^{-3}$ M$_{odot}$, significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining Type Ia SN after SN2018fhw/ASASSN-18tb that shows narrow H$alpha$ in emission in its nebular-phase spectra.
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