Untill April 2nd, 1950, Master was called by His disciples as "Dai Sensei" (= Great Master). Then, following the Divine orientation, He changed His name to Meishu-Sama (The Lord of Light). Mei means "Sun and Moon", and Shu means owner, Lord.
This way, the junction of Sun and Moon (or fire and water) generates the Light, a reference to the power owned by Him through Nyoi-no-Tama, the Light Ball given Him by Kanzeon Bossatsu.
Jinsai.org - Life and work of Master Jinsai