This is a composition of Tallapaka Annamcharya. In
this composition Annamcaharya says that All Mantras are in Venkatesu Mantra
which he acquired as a child.
Naarada chanted Narayana Mantra, Prahlada-Narasimha
Mantra, Vibheeshana-Rama Mantra, Dhruva-Vasudeva Mantra, Arjuna-Krishna Mantra,
and Sage Suka-Vishnu Mantra.
Lord Vishnu is the focus for all the Mantras and the
Vishnu Mantra is the Para Brahma Mantra. Since Lord Venkateswara is
Vishnu, Annamacharya deduces that all Mantras are in Venkatesu Mantra which is
as cool as the Moonlight. (In another composition Annamacharya describes
the qualities of Moonlight, which explains why Annamacharya established
similarity between Venkatesu Mantra and Moonlight)