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Context. Direct imaging provides a steady flow of newly discovered giantplanets and brown dwarf companions. These multi-object systems can provideinformation about the formation of low-mass companions in wide orbits and/orhelp us to speculate about possible migration scenarios. Accurateclassification of companions is crucial for testing formation pathways. Aims.In this work we further characterise the recently discovered candidate for aplanetary-mass companion CS Cha b and determine if it is still accreting.Methods. MUSE is a four-laser-adaptive-optics-assisted medium-resolutionintegral-field spectrograph in the optical part of the spectrum. We observedthe CS Cha system to obtain the first spectrum of CS Cha b. The companion ischaracterised by modelling both the spectrum from 6300 $\unicode{x212B}$ to9300 $\unicode{x212B}$ and the photometry using archival data from the visibleto the near-infrared (NIR). Results. We find evidence of accretion and outflowsignatures in H$\mathrm{\alpha}$ and OI emission. The atmospheric models withthe highest likelihood indicate an effective temperature of $3450\pm50$ K witha $\log{g}$ of $3.6\pm0.5$ dex. Based on evolutionary models, we find that themajority of the object is obscured. We determine the mass of the faintcompanion with several methods to be between 0.07 $M_{\odot}$ and 0.71$M_{\odot}$ with an accretion rate of $\dot{M} = 4 \times 10^{-11 \pm 0.4}$Myr$^{-1}$. Conclusions. Our results show that CS Cha B is most likely amid-M-type star that is obscured by a highly inclined disc, which has led toits previous classification using broadband NIR photometry as a planetary-masscompanion. This shows that it is important and necessary to observe over abroad spectral range to constrain the nature of faint companions

CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion