Context : Direct imaging provides a steady flow of newly discovered giant planets and brown dwarf companions .
These multi-object systems can provide information about the formation of low-mass companions in wide orbits and/or help us to speculate about possible migration scenarios .
Accurate classification of companions is crucial for testing formation pathways .

Aims : In this work we further characterise the recently discovered candidate for a planetary-mass companion CS Cha b and determine if it is still accreting .

Methods : MUSE is a four-laser-adaptive-optics-assisted medium-resolution integral-field spectrograph in the optical part of the spectrum .
We observed the CS Cha system to obtain the first spectrum of CS Cha b .
The companion is characterised by modelling both the spectrum from 6300 Å to 9300 Å and the photometry using archival data from the visible to the near-infrared ( NIR ) .

Results : We find evidence of accretion and outflow signatures in H \alpha and OI emission .
The atmospheric models with the highest likelihood indicate an effective temperature of 3450 \pm 50 K with a \log { \mathrm { g } } of 3.6 \pm 0.5 dex .
Based on evolutionary models , we find that the majority of the object is obscured .
We determine the mass of the faint companion 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 } M _ { \odot } \mathrm { yr } ^ { -1 } .

Conclusions : Our results show that CS Cha B is most likely a mid-M-type star that is obscured by a highly inclined disc , which has led to its previous classification using broadband NIR photometry as a planetary-mass companion .
This shows that it is important and necessary to observe over a broad spectral range to constrain the nature of faint companions .