The changing look of the neutrino-emitter blazar candidate 5BZQ J1243+4043

Context. In recent years, changing-look blazars have called the traditional view of BL Lacs-flat spectrum radio quasars into question within the empirical classification of blazars. Based on the intensity of the optical spectral lines, they appear to transition between the two classes over time. Aims. We focus on the blazar 5BZQ J1243+4043, recently proposed as a promising candidate for the emission of high-energy neutrinos observed by the IceCube Neutrino Observatory and reported as a changing-look blazar in the literature. We study the spectral properties of this blazar, inferring its radiation field and accretion regime properties across different epochs. Methods. This study presents new optical spectroscopy observations of 5BZQ J1243+4043 taken with the Gran Telescopio Canarias. We used this new dataset and two optical spectra available from the literature to investigate the continuum and line emissions and pinpoint the physical properties of the source. In particular, we used the emission lines to probe the accretion regime. Results. The newly collected data for 5BZQ J1243+4043 shows broad emission lines, consistent with the spectrum of the first epoch and the redshift z = 1.5181 ± 0.0002 known from the literature. For the second epoch, the spectrum appears featureless and so, we placed limits on the emission lines and related physical properties. We observed spectral variability for both the continuum and line emissions among the three spectra. Nonetheless, the accretion properties of the blazar generally remain unvaried, indicating that the intrinsic physics stays the same across the three epochs. In the broader multi-messenger context, this suggests that, despite the changing look in the optical band, the candidate neutrino-emitter blazar 5BZQ J1243+4043 is still characterized by the presence of intense external radiation fields and radiatively efficient accretion, typical of high-excitation radio galaxies, which may foster neutrino production.