The old man doesn't have any issues with the Princess, he's just unhappy with Bei Mo's indifference towards Bei Rou.
To know that Bei Rou's grandfather once saved his life, deep down, he considers Bei Rou as his own granddaughter. So he really hopes that Bei Mo would treat Bei Rou like a dear younger sister, rather than be cold and dismissive.
"Grandpa, please don't be angry. I've heard the Princess's temper is not very good, maybe Bei Mo wants to see me, but is afraid of upsetting the Princess, so that's why he says he can't come…" Bei Rou subtly tried to defame Mumu in front of the old man as much as possible.
"Xiao Rou, you are not allowed to speak ill of the Princess, nor say anything bad about the Royal Family!" the old man sternly reprimanded.
He and his family have been loyal to the Royal Family for generations.
Therefore, he cannot stand hearing anyone speak ill of the Royal Family.
