Who is prime t

so i bought minecraft yesterday, when i joined my first world on creative the chat said Prime t joined, press - to open the chat

then i didnt care
today i made another minecraft world, the same random dude joined, it happened like yesterday, he wasnt a real player, he wasnt even on the player list, whenever i switched to the nether or back he would join again
same as joining my own world, someone help, have the same issue please contact me if you know the answer