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We present ALMA Band~7 observations of a remarkably bright galaxy candidateat $z_{\rm phot}$=$16.7^{+1.9}_{-0.3}$ ($M_{\rm UV}$=$-21.6$), S5-z17-1,identified in JWST Early Release Observation data of Stephen's Quintet. We donot detect the dust continuum at 866~$\mu$m, ruling out the possibility that\targb\ is a low-$z$ dusty starburst with a star-formation rate of $\gtrsim30$~$M_{\odot}$~yr$^{-1}$. We detect a 5.1$\sigma$ line feature at$338.726\pm0.007$~GHz exactly coinciding with the JWST source position, with a2\% likelihood of the signal being spurious. The most likely lineidentification would be [OIII]52$\mu$m at $z=16.01$ or [CII]158$\mu$m at$z=4.61$, whose line luminosities do not violate the non-detection of the dustcontinuum in both cases. Together with three other $z\gtrsim$ 11--13 candidategalaxies recently observed with ALMA, we conduct a joint ALMA and JWST spectralenergy distribution (SED) analysis and find that the high-$z$ solution at$z\sim$11--17 is favored in every candidate as a very blue (UV continuum slopeof $\simeq-2.3$) and luminous ($M_{\rm UV}\simeq[-$24:$-21]$) system. Still, wefind in several candidates that reasonable SED fits ($\Delta$$\chi^{2}\lesssim4$) are reproduced by type-II quasar and/or quiescent galaxytemplates with strong emission lines at $z\sim3$--5, where such populationspredicted from their luminosity functions and EW([OIII]+H$\beta$) distributionsare abundant in survey volumes used for the identification of the $z\sim$11--17candidates. While these recent ALMA observation results have strengthened thelikelihood of the high-$z$ solutions, lower-$z$ possibilities are notcompletely ruled out in several of the $z\sim$11--17 candidates, indicating theneed to consider the relative surface densities of the lower-$z$ contaminantsin the ultra high-$z$ galaxy search.

ALMA FIR View of Ultra High-redshift Galaxy Candidates at $z\sim$ 11-17: Blue Monsters or Low-$z$ Red Interlopers?