Niranjano nirakara, eko devo Maheswara
From where is this mantra taken?
It seems to be related to the mahamritunjaya Mantra.
The "निरंजनो निराकार एको देवो महेश्वरः (Niranjano nirakara, eko devo Maheswara)" comes from an ancient Sanskrit tantric text named Shiva Swarodaya. This is found in 6th shloka of the text which contains total 395 shlokas:
तत्वमेव परं मूलं निश्चितं तत्त्ववादिभिः।
तत्त्वस्वरूपं किं देव तत्त्वमेव प्रकाशय।।5।।
The tattwas (elements) are the primal cause as ascertained by the exponents of the tattwas. O Lord, what is the nature of those elements? Kindly reveal that to me.
There is only one birthless and formless supreme existence from which evolves akasha (ether element), and from akasha comes vayu (air element).