How should i work with player that doesnt take it seriously at all?

Hello my Friends and fellow DM’s. I am a DM myself and i really need a advice.

I play with my Group a Campaign with FATE Rules in a Postapocalyptic Dark and Gritty Homebrew world wich is mainly Horror based, written by a good friend and myself about 2 years ago. Wich i also DM’d for another group.I give my group alot off freedom what they do and how they handle the Storys since this is my main goal with telling the Story of the world itself.

I play with with 5 player (soon 6) while 3 off them take the world and the Lore seriously, made beautifull characters and gift me with their own creativity and Ideas for the whole Plot, i have 2 Players that are pretty hard to handle.

One of them is completly passive when he is even present in a session and almost doesnt listen to me as well. He only plays and brings himself into the Game when i especially talk to him and ask him what would be the best idea for the sake of the Group in the current Situation. It is pretty hard to handle this player cause i feel like i talk to a literal brickwall. But he is not the hardest thing.

One of the other players doesnt take anythign seriously at all, while i don’t mind one joke or too, since i myself make sometimes references as a DM for a little laugh to enlighten in some situations the gritty atmosphere, this player completly overdoos it.

He literally wants to hug and seduce literally everyone may it be Bandit, or Mercenary, Mutant or Zombie. Or even something bigger i made up. He brings the Group into a really dangerous situation and then runs away and let the other players Handle a dangerous Siutation he himself made up. Asks me the whole time if he can befriend a Mighty Mutant i made up in the Lore as one of the Dangerous and most hostile thing in the whole Lore, and even if a NPC said to him that this is one thing that is impossible if not completly suicide.

I tried the last session to give him a little kick, i made up a really big treath, a Lore heavy mutant that is for the group in their current situation impossible to handle or even to fight. The Group saw this Mutant and this one player approaches him directly, i gave him several warnings like “a long forgotten instinct that warns him from a mighty Predator.” and several NPC’s that scream too him that he should run since he is literally killing himself. He wanted to gave the mutant a handshake, and well i have to do what is more lore apropriate, the mutants head splitted like a lotusblossom and litterally bites the player in the whole arm, infecting him with a dangerous Curse.

But well you guess it, even that he didnt took seriously, not even gave any sort of “damn i fucked up” he played it out like it was nothing. And literally shrugs at me when i tried to tell him the danger of his actions.

The Groups meeting with this mutnat was supossed to be a lore heavy and important for the later Story, and it got completly fu**ed up.

I grow tired of this shitshow cause i made this work and it gone like this, even if i asked mutlible times “Can we please concrentrate us and take it mroe seriously”, this one player completly ignores it and i literally don’t know what i can do.

I trought of it to confront this player outside of the Sessions and ask him to take it more seriously cause i grow tired, but i also don’t want to loose my professionality if you understand me.

I really thank you for your advices, caause i’m desperate and i don’t know anymore what to do.

Happy Rolling my friends!