We report the discovery of a mysterious giant $H_{alpha}$ blob that is $sim 8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxy merger, identified in the first-year SDSS-IV MaNGA data. The size of the $H_{alpha}$ blob is $sim$ 3-4 kpc in radius, and the $H_{alpha}$ distribution is centrally concentrated. However, there is no optical continuum counterpart in deep broadband images reaching $sim$26.9 mag arcsec$^{-2}$ in surface brightness. We estimate that the masses of ionized and cold gases are $3.3 times 10^{5}$ $rm M_{odot}$ and $< 1.3 times 10^{9}$ $rm M_{odot}$, respectively. The emission-line ratios indicate that the $H_{alpha}$ blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the $H_{alpha}$ blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this $H_{alpha}$ blob include AGN outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145, or an extremely low surface brightness (LSB) galaxy. However, the stripping scenario is less favoured according to galaxy merger simulations and the morphology of the $H_{alpha}$ blob. With the current data, we can not distinguish whether this $H_{alpha}$ blob is ejected gas due to a past AGN outburst, or a special category of `ultra-diffuse galaxy (UDG) interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145.
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