Q. In order to be yotze with maror, how much of the flavor of the horseradish should be left, if it was left open?
A. Poskim disagree if it is preferred to grind the maror before Yom Tov and leave it well covered until the night, or not. (Magen Avrohom 504: 7, Chaye Adam 120: 3, et.al/). When Pesach is on Shabbos, all agree that it should be grated before.
If the flavor of the horseradish became weak, Poskim maintain that one complies, as long as some flavor remains. (See Hilchos Pesach 29: 2). Horav Shlomo Miller’s Shlit’a opinion is similar. (See question 2145)
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a