Matad Ambi (as named internally) is a melancholic-sounding, ambient theme in Metroid: Samus Returns. It is heard after Samus drains any cavern containing purple liquid by using a Chozo Seal. It will play immediately after the Matad Jinchozo jingle (which plays when the liquid lowers) stops, and cease when Samus boards the elevator to the next area. It is always heard in the drained caverns on subsequent visits. It was composed by Daisuke Matsuoka. The word "Matad" is a reference to "Project Matadora", the development codename for Samus Returns, while "Ambi" is most likely short for "ambient" or "ambience".
The theme can be heard here.