The words ‘grace’ and ‘mercy’ are synonyms in English. Prabhupada did not use these words in a technical, Christian theological sense, but rather in the simple, general sense of the words.
Thus, mercy means compassion, kindness, or forgiveness towards someone whom one has the power to harm. Grace means the same, with the sense that the mercy is given even when the recipient is not strictly deserving.
In fact, important Sanskrit words like prasada often do not have one absolutely identical English equivalent. Thus by giving various English translations in different contexts, Prabhupada is providing a larger, more complete picture of the range of meaning of the original Sanskrit word.