|This article is written from the Real Life point of view|
It is heard during the first part of the Nettori battle, when the boss is in a form resembling a Chozo Statue. When Nettori turns into a Core-X, the theme transitions into VS. Core-X, Chozo Statue Second Form.
It is an eerie theme, based largely around ascending arpeggios performed by a harp or some other plucked string instrument. The theme features tambourine-like beats, heavy drumrolls, and slowly trilling woodwinds at regular intervals. This instrumentation evokes the jungle atmosphere of Sector 2. The song is rather short and loops around every 17 seconds.
It is similar to the Mini Boss Confrontation BGM (Spore Spawn, Botwoon) theme from Super Metroid; Spore Spawn boss in that game also released spores, much like the Samus Eater Buds during the Nettori battle. Hamano composed for Super Metroid as well, so it is possible that the similarity was intentional.
The theme can be heard here.