When the rice thrown by Nityananda Prabhu touched His body, Advaita Acarya thought Himself purified by the touch of remnants thrown by Paramahamsa Nityananda. Therefore He began dancing.

PURPORT: The remnants of food left by a pure Vaisnava are called maha-maha-prasada. This is completely spiritual and is identified with Lord Visnu. Such remnants are not ordinary. The spiritual master is to be considered on the stage of paramahamsa and beyond the jurisdiction of the varnasrama institution. The remnants of food left by the spiritual master and similar paramahamsas or pure Vaisnavas are purifying. When an ordinary person touches such prasada, his mind is purified, and his mind is raised to the status of a pure brahmana. The behavior and statements of Advaita Acarya are meant for the understanding of ordinary people who are unaware of the strength of spiritual values, not knowing the potency of foodstuffs left by the bona fide spiritual master and pure Vaisnavas.

Srila Prabhupada, Sri Caitanya-caritamrta Madhya-lila 3.96