Is saying, "May Allah accept your prayer" after the worshiper finishes his prayer considered an innovation (bidʻah)?

The default is that it is recommended for a Muslim to supplicate for his brother in his absence, based on the Prophet's (peace be upon him) saying: "The supplication of a Muslim for his brother in his absence is answered. At his head there is an angel appointed, every time he supplicates for his brother with good, the appointed angel says: 'Ameen, and to you the like thereof.'" This is not described as an innovation (bid'ah) unless the supplicant believes in a specific sunnah for his supplication on this occasion.