Interaction between Subtle Spell, Iniciative and Alert Feat


I’m running a game and I’m trying to gank a few of my players with a Sorcerer with Subtle Metamagic. However, one of the players has the Alert feat which states he cannot be surprised.

My question here is: can I cast subtle hold person with the sorcerer without any interference from the player with the Alert feat? Or does that person has the opportunity do so something about it since he cannot be surprised? Let’s say the Sorcerer is hiding in his hand his arcane focus (a small pearl).

Can I do this without asking my players to roll for initiative?

Thanks!