I'm slightly confused by the translation, but if I understand correctly the guildmaster is Marissa's grandfather, and it's his son and his son's wife who has an issue with Marissa adopting another child. Marissa did call him grandpa earlier, but Millie also called him grandpa instead of great-grandpa. I also understand better what was said in the previous chapter; so there never was an issue with Marissa's grandpa from the guild, but with Marissa's parents running the Roseretta trading company.
It's a bit odd that the guildmaster son's decision would dictate his behaviour towards his granddaughter. He's the elder and still working in a position of authority so he is independent and should have been able to overrule his son's decision or at least ignore it for himself.
It's funny seeing he has some sort of ninja, with magical stealth powers, working for him who claims to have taken the necessary steps to push Marissa's family into registering their recipes. The one who suggested this was the prince, who also has someone working for him and monitoring the place... And all the ninja was doing was apparently being a customer at the inn, or maybe they did more.
I'm not sure what the guildmaster means when he says Millie is a ticking time bomb since there are several elements. There's the recipes (and the disrupting knowledge behind them), with the wealth that comes with them, that have already attracted the prince's attention and the jealousy of rival inns. There's the large amount of magic that could be dangerous. There's the unknown origins, large magic and original hair colour (though I'm not sure he knows about that since millie had no visible hair as a baby and was kept hidden as a child), that suggest she is a noble's child which may also become a source of trouble.